I gotta go with this for the best oD pedal.
I've been using these for years. Even with the original tube(Chinese 12ax7) these pedals are silky smooth, but when you get into changing the tubes you really find the versatility of this pedal.You can cut the overdrive by going to either a 12au7 or even a 12at7. Every time you change tube, the characterics of the overdrive changes...Personal favorite-Ei/12ax7 & Tesla/JJ 12ax7>>>>>>worst by far- SOVTEK.
I also like the newer version(wallwart).The older with the transformer inside tend to be noisy.

the older one had a transformer built into it you say?... do you have pic or a description of what it looked like? I have a couple of these pedals and they are truuuuelly great pedals. Not an overwhelming, take over overdrive. I too have had the opportunity to try many tubes through the unit - 12AX7, 12AX7WA, 12AX7WB, 12AX7WXT+, 12AX7R(I believe), 12AX7A, and of course the Ei ECC83, which I agree sounds the best for overall gain, tone, noise, etc effect. Actually, swagon have you tried the Hughes and Kettner Tube Factor? While I think the Chanlder Tube Driver is great the H&K is scary... if not as good as the Chandler even better no bar none! Extremely transparent as we all say and very true to your tone. You have to tinker with the unit a bit to replace the tube in it as it is not readily accessible like the Chandler unit.