Replaced the valves in my terror yesterday so thought I'd share my thoughts. Got two Tung Sol 12AX7's from Watford Valves.
The main reason I decided to swap them was I found the low end to be a little muddy and undefined, particularly right down at the lowest frets. I have to say that right away I noticed a difference. It's not a major tone change, it's still the same great Orange sound, but everything sounds a little 'tighter', if that makes sense. There's definitely an added brightness, and the mids sound perhaps a little more aggresive, if that's possible!
I went for the Tung Sol valves in particular as I seen someone on Talkbass had mentioned that they work particularly well with an aggressive fuzz/OD in the terror. This is most definitely true! Higher gain settings on my ODB-3 sound even better now, with the clearer low end and brighter sound making the notes jump right through instead of kind of lurking around underneath the effect. Another plus it that the amp seems more responsive to the eq on the Boss pedal as well. The smallest tweak of the treble knob gives noticeable and usable change in tone, which is brilliant. (I've also just received an MXR M108 eq pedal so this bodes well!)
I haven't had the chance to properly put it through its paces yet, this was just what I found after a couple of hours of jamming in my house with my guitarist (got a band practice tomorrow though so will have the chance to see how it reacts to higher volume/gain settings), but so far it seems like a very good move. For less than £30 inc delivery I'd recommend it.