By now, most HeadFiers have probably read Tyll's review of the Sennheiser HD660S over at InnerFidelity If not, you should certainly do so as soon as possible. Make sure to watch the accompanying video as well.Funny story - the HD660S that Tyll reviewed was actually sent to me first, due to a misunderstanding. I asked for a review unit so I could cover it at Digital Audio Review, but the PR guy I was working with thought it would be for InnerFidelity (since I contribute to both sites). I definitely wanted Tyll to get first crack at it, and the PR folks didn't have another unit for Tyll, so I shot the package his way without even opening it.The following week, I was able to get my hands on another HD660S from a friend. After spending time with it, I have to reluctantly agree with Tyll - this is a good headphone... perhaps even agood headphone, all things considered. But it's not an evolution or improvement on the existing HD600/HD650 as we had hoped. Net result? Disappointment, as you can clearly see in Tyll's video review.HD660S will likely still be a modest success for Sennheiser. But I predict many enthusiasts will find themselves underwhelmed for the price, and ultimately still waiting for a true HD600/HD650 successor.Meanwhile, Massdrop has a project in the works that may help. It probably won't helpall that much, but might be just the thing for enthusiasts who feel disillusioned by an underperforming $499 HD660S.The topic of discussion here is actually the new Massdrop x Sennheiser HD58X Jubilee. If you're reading this, you've probably already read or watched something about it from Jude, so I'm not going to repeat all that info. The Massdrop page also has the details so head over there and have a look. The most important detail, in my humble opinion, is the $149.99 price tag. Massdrop's other collaboration with Sennheiser - the HD6XX - has been a huge success at $199.99, and this new offering aims to continue that winning streak.A few important points before I get into the sound aspect:*In case there was any doubt, I can confirm that the HD58Xin fact use the new 150-ohm Sennheiser driver "platform" - which looks undeniably like the one found in their HD700 (but don't hold that against it). I tried to capture pics but wasn't completely successful due to the grill... it was far easier to see with the naked eye though. Despite the ease with which these cans can be disassembled, I wasn't comfortable taking apart the review unit just to get a better picture - it's obviously the new driver anyway, so why take the (small) risk?*I've confirmed with Massdrop that the HD58X uses the "older" style pads, rather than the new and slightly different HD660S variation.*The headband pads are the "4 lumps" style as seen in HD580/HD600, rather than the longer pad with a split in the middle used by HD650/HD660S. I'm indifferent about this choice... both styles are exceedingly comfy to me, but you may feel differently.*Finish is a bit of a mash-up. It has the smaller, offset "Sennheiser" text on top of the headband, and is all black like HD660S. But where that model has a matte finish, the HD58X has a glossy black, similar to the somewhat glossy finish on the HD650. It looks quite fetching though seems to show fingerprints more than any other variation.*My review unit, which Massdrop says is the final design, says "Made in Ireland". Just in case anyone was concerned about Sennheiser pulling an AKG by switching production to China or elsewhere.*The cable is just the standard 6-foot HD650-style cable with a 1/8" termination. Meanwhile the HD660S comes with two cables, one of which has the 4.4mm balanced Pentaconn termination. It makes sense that Massdrop removed the extra bundled cable considering the price discrepancy. Plus I'm not really a fan of the Pentaconn style anyway (seems unnecessary), and that cable is too long in the HD660S bundle. So personally I don't miss this at all.Anyway.Getting back to the drivers: Massdrop tells me this is the same "platform" as seen in the HD660S, but not quite identical. It's in the same "family of drivers" but has "different specs". Not sure how exactly they differ. The core of this driver architecture is an aluminum voice coil and new magnetic structure, which should result in more accurate transient response and faster decay - according to Axel Grell of Sennheiser.My listening confirms that - at least to some extent. The HD58X Jubilee is indeed a bit faster than my well-worn HD650 and also a newer HD6XX. Sound is more immediate, more extended, not as warm/smooth overall. The "veil" is not so prevalent here. Perceived bass impact is... different. On the one hand it seems to kick a little harder down low, but then again it has reduced mid and upper bass so the overall feeling of warmth is reduced. The result might be the impression of improved kick when really it isn't much different. Soundstage seems wider - which I appreciate - though still lacking compared to an HD800. Particularly in depth which is not a strong suite here. Still, if you've always struggled with the HD650 sounding a bit compressed in terms of staging width, you'll appreciate this upgrade.The HD58X is also (moderately) easier to drive. The Cowon Plenue 1 or Sony ZX2 seem more comfortable with the Jubilee than the "classic" models, and reach higher volumes more easily; which should result in superior battery life overall. I even find that an iPad or Galaxy S7 Edge does a better job with the Jubilee in terms of drive, though I do tend to miss the forgiving nature of the HD650 with those sources. In general though, if you listen primarily from a DAP, the 150 ohm HD58X is often the better match.The flip side of that is I don't feel it scales as well on nicer desktop gear. HD650 has practically limitless potential, while HD58X seems to top out on moderate systems. The Jubilee can't really show much improvement beyond something like a Cavalli CTH or Rupert Neve RNHP. Ditto for DACs - your really-high-end source is sort of wasted here, so you can comfortably top out under $1K.Is this a good thing or bad thing? Depends on perspective I suppose, but for me the increased scalability of the old models is more beneficial than the reduced amping requirements. I mostly listen at home on big gear though... others may feel differently.It's very important to remember that this is meant to be a spiritual successor to the old HD580 Jubilee, which was essentially an HD600 before the real HD600 actually launched. So we're talking far more similarities to the HD600 than HD650. I no longer own an HD600 at the moment so unfortunately I can't directly compare.What Icompare is the HD660S. And honestly I hearminimal differences. Despite the pads being very slightly revised, and the drivers supposedly not being identical, I still don't hear a variation larger than what you might find between an older HD650 with flattened pads and a crisp new HD6XX. Which happens to be the exact scenario I have here. Or, to put it another way, the difference between HD58X and HD660S is less significant than the difference between HD600 and HD650. Also smaller than the differences between some of the AKG models, like switching from K702 to their 65th Anniversary Edition. If I must name a difference, I'd say the HD660S has more refined treble, but it's not at all a significant difference - I barely notice it for the most part.Honestly, I'm disappointed in what Sennheiser has done with the HD660S. It's not a terrible headphone by any means, but it feels like a missed opportunity, and in a world where the HD6XX sells for $199, it seems pretty expensive.On the other hand, I'm actually excited about the HD58X Jubilee. Yes, it's a little on the brighter side compared to HD650/HD6XX. And no, it won't scale to the same heights. But for a penny less than $150 it's an absolutely compelling release. If I'm being frank, I can't believe Sennheiser is letting this happen considering the similarity with HD660S and the difference in price. But I suppose Sennheiser will sell a ton of the more expensive model through outlets like Crutchfield, Sweetwater, Guitar Center, etc. Meanwhile, the enthusiast crowd will snatch up the HD58X for a steal of a price.If you already own an HD600, this still might not add much to the conversation. If you own an HD650 and want a somewhat brighter, more expansive sound, this is worth a try for sure. If you don't own any of these, it's a toss up between HD6XX and HD58X Jubilee... I can think of situations where either one wins out, so there's no universal recommendation. Regardless, Massdrop should win some fans here.