I have had the WA2 for a couple of months now and it's been a journey to get great sound. Without question, the device is excellent (mine has the Black Gate caps and stepped attenuator) but you need the right tubes to get a great sound.
What is great sound? Well, for me it's sound that is musically enjoyable with natural sounding harmonics and no harshness. It's hard to get that without losing detail and requires careful experimentation. I can't tell you what you will like but I'd like to share what I believe to be a great set-up.
First I'd like to say that changing any one of the three tube types (rectifier, power tube or output tube) can change the sound from great to bad. It's really that sensitive. That also means you may not want to blow $200 or $400 on some NOS output tubes only to find you're too broke to buy the rectifier and power tubes you need to get the sound you want.
I have bought a lot of tubes and each time only changed one type, taking notes on the sound, then changing again. I will not detail every combination and my notes but here is a list of tubes I have tried so far. The good news is that there is virtually no correlation between price or new versus NOS and sound quality. This gives you the chance to get a great set-up without breaking the bank.
My original goal was to get the cheapest set-up that sounded really good and to do it using only current production tubes because that would mean I could always get the tubes I needed to have great sound. NOS tubes cost more and there's no guarantee you'll still be able to find your favorites ones a few years from now. At the end I will recommend the cheapest set-up that sounds satisfying (to me) and then the best set-up so far irrespective of price. Note however, that I have not yet tried the super expensive output NOS tubes out there.
Test set-up is a PC, ASUS Xonar STX Sound card, Harmonic Technology TruthLink II interconnects, Woo Audio 2 and Beyer Dynamic T1 headphones.
Tesla 6CA4 / EZ81 $20 each (these top three all sound great)
Electro Harmonix 6CA4 $14 each
JJ EZ81 / 6CA4 $12 each
Mullard 6V4 NOS matched pair Woo upgrade $70 each (lack dynamics, boringly smooth)
Tung Sol 5998 NOS Best $115 each Magical combination of control, harmonic richness and transparency
Tung Sol 7236 NOS $125 each (not nearly as good as the Tung Sol 5998)
Sylvania 7236 NOS $60 each (Woo upgrade) adds punch and control compared to the Sovtek
Sovtek 6AS7G $50 each adds richness compared to the Sino
Sino 6AS7 $14 each surprisingly satisfying for the price
RCA 6080 NOS (stock Woo) sounds like a thick veil stands between you and the music
JAN-Sylvania 7308 NOS Best $60 each
JJ E88CC / 6922 Gold (great value) $25 each
Philips 7DJ8 / PCC88 NOS $20 each
Not musically enjoyable / harsh
Genalex - Gold Lion E88CC / 6922 $45 each
Tesla E88CC / 6922 Steel Pin $25 each
Hand-pick matched pair Philips 6922 JAN Woo upgrade $20 each
For the rectifier tubes, you can get great performance for low expenditure. NOS tubes may be a waste of money.
For the power tubes they don't seem to make the sound good or bad but rather change the degree of control, dynamics and richness. The Tung Sol 5998 are king and make a huge difference. For me, you have to have those or you are not getting the most from your WA2. They give such control with transparency and great, natural harmonics. They just sound so "right".
There are a huge number of options for output tubes. The JJ were my favorites and are satisfying, not to mention inexpensive. A great buy. Then I tried the Sylvania 7308 and they are as close to perfect as I can imagine without having heard the more expensive options.
Least Expensive Good Sounding Combination
JJ EZ81 / 6CA4 $12 each, Sino 6AS7 $14 each, JJ E88CC / 6922 Gold $25 each
Best Sounding Combination
Tesla 6CA4 / EZ81 $20 each, Tung Sol 5998 NOS Best $115 each, JAN-Sylvania 7308 NOS Best $60 each
I spent quite a bit of time and money looking for that magical combination and I hope someone else can spend a little less of each on their journey having read this. If you do by a WA2 then you really need to change the tubes to get good sound.