I recently built my TW Express and it is really a fire breathing monster!
Have experimented with several tube sets including: 6v6, 6CA7 and EL34s.
Judging from the TW clips on YouTube, the original TW cleaned up amazingly and could go from clean to insane with the swell of one's guitar volume. The main problem with the TW Express by default is that it is way over gained, there is no possibility of cleaning up or dialing in any workable tone past volume one (7:00). I would seriously bring this to Nik's attention however I have never played through an original TW and Nik is god in some circles
Here is an original 1989 TW:
I was a little discouraged until I experimented with several preamp tube configurations I have found something that is more versatile from liquid lead to clean. Try using a Tung Sol 12ax7 in V1 and an EH12AT7 in V2. This brought down the gain by 40%. You can further reduce the gain using 12AU7. Check out: http://thetubestore.com/gainfactor.html The newly designed TungSol 12ax7 have no discernable microphonics! You can tap the EH or JJ 12ax7 and hear how microphonic they really are.
In my experience, I'd further recommend humbuckers as single coils don't drive the TW that well.
Having built the TW I later installed the PVIMV Type II, I personally like the sound of the 6CA7 biased at 40ma. They have a lot more bottom than the EL34s and sound great through my Marshall 4x12 with greenbacks. This is indeed TW nirvana.