First post...just joined the WA7 club.
A bit of back story. Mid way through last year, largely due to glowing reviews on head-fi, I made the fateful plunge and purchased the Sennheiser HD600 along with a Maverick Audio TubeMagic D1 amp/dac. At the moment of purchase, it seemed like utter madness. However, I had recently moved in to an apartment where I could not longer use my loudspeakers and I was really missing my music. Desperate times. As many of you have experienced, the purchase proved to be more of a revelation than a mere frivolity.
Initially, I was very happy with the combo, as it was significantly ahead of any previous headphone setup I'd heard. There were some weaknesses that were immediately apparent however. Perhaps most notably was a lack of weight in the lower frequencies. I assumed this was largely part of the sound signature of the headphone and that there wasn't much I could do about it. After all, the Maverick Audio amp seemed to have more than enough power to drive the headphones, and the signal came across as very clean. Like many who probably browse these forums, my first impulse to was to investigate a new headphone purchase. I began reading various headphone threads like a mad fool, learning about the various sound signatures available and considering where I should turn next. It quickly became apparent that half of the discussion surrounded which amps matched well with each particular headphone.
So, while it seemed like it'd be less "fun", I decided to tackle my amp/dac situation first. Pairing a 4 figure headphone with a cheap amp just didn't make a whole lot of sense. Unfortunately, I had to make another blind purchase which is rather nerve-racking. The WA7 caught my attention for a number of reasons. First, it seemed to pair fairly well with a wide variety of headphones. Second, it seemed to have enough power (as well as flexibility) to address pretty much anything I'd probably buy in the near future. Third, the feedback was overwhelmingly positive and folks seemed to perceive it as a solid value.
My unit arrived three days ago and I've been glued to my headphones getting to know the amp. Based on the comments in this thread, I've been using the Electro Harmonix tubes. To be blunt, I'm absolutely blown away by the difference in sound quality. I'd read about the "scalability" of the HD6xx series (perhaps the 650 more than the 600), but wasn't entirely sure what that would translate to in this case. Verdict: It's almost like having a different headphone.
The overall tonality is significantly smoother and transparent with less grain, and there is no longer any harshness in the treble (something I had attributed to the headphone initially). Although it seems a bit contradictory to the "smooth" descriptor, there also is a significantly heightened sense of dynamics throughout the entire sonic palette - really beautiful. The soundstage has both widened, increased in depth and gained a sense of air, creating a far more cohesive image. The bass is weightier, and more textured while still effortlessly flowing in to the midrange (I'd still like better extension for some electronica, but it's way better than it was).
All in all, I am just stoked. I feel that it's been money very well spent and it's far exceeded my expectations. Truly awesome. As far as headphones go, I am now rediscovering the HD600 but plan to also add an ortho to the mix very soon. Thanks Woo and thanks to all of the folks in this thread for providing quality impressions/information!
Jack of Woo indicated they do not expect a noticeable improvement using usb power filters which I took to mean they already did due diligence in isolating and filtering the DAC reference but if you are looking for piece of mind a cost effective solution for better power isolation and filtering is http://hifimediy.com/index.php?route=product/product&product_id=122. I use one and it seems to work well though I cannot say I notice an improvement for me it is just piece of mind.
