Pros: Soundstage, Neutrality, Price, Size, US Made
Cons: It wasn't free?
I had been looking for an amplifier for my Sennheiser HD650 headphones and tried many amps that cost much more than the Valhalla over the last 6 or so years. It's almost like they designed this amplifier to bring out the best of this particular headphone. If you are also a 650 owner and have always thought that they are a bit "veiled" in the mids/highs, and like me have done the mods (foam, cable) and you still feel that after all that's been done that the 650's still sound a bit "dark", then do yourself a favor and check this amplifier out. Seriously, before they realize they can be charging a bit more for it.
I had started my quest for the perfect amp for my 650s with Solid State designs, and before the Valhalla, it was paired with the (awesome) Headphone amp on my Crane Song DAC; we're talking about a mastering-grade DAC and monitor controller here folks, and it costs nearly three grand. I still felt that the mids and highs were a bit scooped. A fellow audio engineer suggested I try tube amps. I have heard some very good (and very expensive) tube amps with the 650s and a lot of them (interestingly, most of the really expensive ones) colored the sound a bit too much for my taste. I love the neutrality of the low end that I get from my SS amp, and all of the (really expensive) tube amps I tried colored the sound so much (and then I remembered that we audio engineers understand that if we want neutrality from amplifiers, that Solid State is the way to go). I decided to end my quest realizing that I already have a high-end SS amp in my Crane Song DAC, and simply felt that the 650s could not sound any more neutral and wrote the 650s off as a pair of headphones that simply could not be linear, as tube amps had let me down in the sense that they aren't neutral and colored the sound a bit too much to use in a critical listening situation (most audiophiles are fine with this because coloration is more accepted for leisure listening than it is for critical audio work, where neutrality is most desired).
I found out about Schiit and the Valhalla amp a few months ago, and several reviewers said that it sounded neutral for a tube amp, and this is what honestly got me interested. Again, a large portion of the audiophile community may not want this, and I've already seen reviews/comments from people who say to try different tubes because the amp isn't colorful enough. This got me even MORE interested. I got my amp about a month ago and all I have to say is: WOW. The sound stage is amazing. I'm hearing more details in the mids and highs that I had not heard from my 650s, all for under 400 bucks. Obviously I'm not interested in more coloration and love that Schiit offers a full set of replacement (matched) tubes for $40! I could not wish for a better tube amp for my 650s. I have absolutely zero interest in searching for matched NOS tubes, especially knowing that there will never be another set that's going to sound the same. I won't have these issues with the matched tubes that Schiit sells for this amp (and if there is a difference, I'm sure it won't be that drastic). I plan on ordering a couple of spare sets to have (I'm not going to wait until the tubes that came with the amp wear out before I order a few sets to have for backup).
For the audio engineer that loves the sound of the 650s but wants a more linear sound from this model headphone, this amp is a dream (and a steal under $400).