I cover a wide range of high-end portable music players on this blog. I've favorably reviewed the $200 FiiO X3 and $350 FiiO X5 players, and a bunch of portable digital converters/headphone amps you can use with your phone or computer. The best-sounding one so far was the $700 Centrance HiFi M8. That one took sound quality from a battery-powered device to a new high; the HiFi M8 is the one to beat.

HiFiMan

That's all under-the-radar audiophile stuff, though. Neil Young's upcoming Pono Kickstarter project is reaching a broader, more mainstream audience. I'm guessing most Young supporters think his high-resolution music player is a brand-new idea, but HiFiMan has been making awesome-sounding music players since 2007. The current flagship model, the HM-901, sounds better and more powerful than any other portable (battery-powered) music player I've tried to date.

I find the HM-901's button layout and click wheel are easier to use than the FiiO and Astell&Kern players' controls, but the HM-901 is bigger and bulkier than those sleek players. The HM-091's user interface takes some getting used to, and there's some lag time between when you press a button and when the player executes the command.

The HM-901's modular construction makes it possible to upgrade the sound with the choice of plug-in amplifier cards. The player has low and high gain settings to accommodate a wide range of in-ear and full-size headphones. I really like the analog volume control, as it makes it so much easier to dial in the exact volume than more typical up/down buttons. There's no built-in memory, but you can pop in SD cards of up to 128GB. The HM-901 plays MP3, AAC, AIFF, FLAC, ALAC, APE, and WAV files, up to 24-bit/192-kHz resolution.

The HM-901 partnered with the Audeze LCD 3 headphones knocked me for a loop; power and dynamics were extraordinary.

Amy Winehouse's funky version of "The Girl From Ipanema" connected on a very different level than what I've heard from other portable players -- Winehouse's performance grabbed me by the throat.

The Black Keys' pounding rhythms are something else, and the HM-901 can play 'em louder than any other portable player or digital converter/headphone amp I've tried. It never seems to be working all that hard...other portables run out of gas long before this one.

Once you get used to listening to the better-mastered high-resolution 192-kHz/24-bit files, MP3s and standard-resolution FLAC files can sound a little bland. That said, I don't have that many high-resolution files so I mostly listened to standard files, and confined my comments to the HM-901's sound with them, with the upgrade balanced amplifier card installed.

Comparisons with the now-discontinued Astell & Kern AK120 music player ($1,299) didn't take long,: the HM-901 is considerably more transparent and powerful, there was no contest, even with the better in-ear headphones, like the Sennheiser iE 800 (video). The performance gap only grew wider with hard-to-drive headphones like my 250-Ohm Beyerdynamic T90 . If you're the type that likes to mash the throttle on Nine Inch Nails or ZZ Top tunes you need the HM-901. The Astell & Kern players have touch screens, they're a lot smaller and better looking, but if hot-blooded sound quality is what you're after the HM-901 wins hands down.

As for the Centrance Hi-Fi M8 amp I mentioned earlier in this review, it's far ahead of the AK120, for little more than half the AK120's price, especially when used with hard-to-drive, full-size headphones like those T90s. The HM-901 takes the sound to the next level, it's simply more powerful and has superior resolution of fine detail. If you're using only in-ear headphones the differences between the various high-end digital converters and players will be much smaller; just get the HiFiMan M8 and don't look back. With full-size headphones the HM-901 pulls ahead.

The absolute best sound I heard from the HM-901 happened when I used it with the HiFiMan HE-560 headphones ($899), hooked up with a HiFiMan "balanced" cable. Transparency and the wide-open imaging jumped up a few points. The HiFiMan combo is a state-of-the-art music system.

So if you're ever lucky enough to hear the HM-901, make sure to listen with a great set of headphones. Otherwise you'll be wasting your time: this player only makes sense for serious headphone enthusiasts with at least one pair of really high-quality headphones. The HM-901 is so good that once you've heard it you'll have to buy it.