With the HD 6XX, Sennheiser kept the same audio quality from the HD 650 and made mostly cosmetic changes. For starters, the HD 6XX headphones are blue instead of gray and they come with a shorter 6-foot cable. This new model also uses an 1/8 in. plug rather than a 1/4 in. Don't worry, there's a 1/4-inch adapter should you need it. That's really about it in terms of changes and you can expect the same stellar audio quality that those open-back headphones served up a few years ago. In fact, the folks over at Head-Fi ran a number of tests on both models to make sure. The HD 6XX displayed nearly identical performance to the HD 650 in terms of frequency response and total harmonic distortion.

I've been using the HD 6XX for a few days and I can attest to the remarkable audio quality. The tone is warm and the sound is extremely clear, offering high enough resolution to pick out subtle parts of songs that regular ol' earbuds and everyday headphones gloss over. Through a range of genres including bluegrass, hip-hop, metal and more, these headphones handle highs, mids and lows with ease and you can crank the volume way up without having to worry about any distortion. There's a good amount of bass here, but it blends in with the overall sound nicely. It's by no means overpowering and sounds more natural than a lot of headphones available today. With the open-back design, you'll want to use these at home or in a private office as they won't block out outside noise and the people around you will be able to hear whatever you're listening to.

The HD 6XX is also really comfortable to wear. Soft earpads and a headband that doesn't pinch in on your head make long listening sessions enjoyable rather than an endurance test. If you find yourself needing to tune out the work for a while, you'll be able to comfortably do so with these headphones and your audio setup.

While you can use the HD 6XX on its own, the headphones perform best when paired with an amp, high-res audio player or some other audiophile-grade gear. I've been using them with the Apogee Groove: a combination headphone amp and DAC (digital-to-analog converter). The $300 USB accessory helps these new Sennheiser headphones really shine, offering a boost to the volume so that you can hear all the grit and nuance in the Led Zepplin BBC Sessions or Miles Davis' Bitches Brew. With the lower price, you could pick up the HD 6XX and the Groove for the same total investment as the original HD 650.

So, what's Massdrop? The site specializes in getting members gear at a discounted price. Products are selected from a range of categories to gauge interest. If enough people indicate that they might purchase an item, the folks at Massdrop schedule a "drop" so that members can purchase it. Due to the popularity of the HD 650, there has been plenty of interest in the HD 6XX. After a certain number of people buy a product, the price drops so you know you're getting a good deal. In the case of the HD 6XX, pricing starts at $250 and when it hits 50 units sold, the $50 discount kicks in.

The HD 6XX will be available for purchase at Massdrop at 9AM ET today. You'll want to act quickly though as these headphones are limited to 5,000 units. If you're hoping to snag a pair for the audio geek in your life for the holidays, you'll have them in time for Christmas. They're scheduled to ship December 15th. For more details on how to nab 'em, head on over to the product page on Massdrop.

Update: This post originally stated that the HD 650 had been discontinued. That is not the case. We've updated the text to reflect this information.