Hey, I can help you out with this. I'm huge on Greenbacks. I use Marshall G12c greenbacks that were modeled on Marshall's old 60's era greenback that they use as a reference speaker. This speaker was sent to Celestion to be re-engineered, and they used them in their Hendrix reissue cabs and Vintage Modern Cabs. They're fantastic sounding Greenbacks. When I want a slightly more 80's metal sound, I've thrown in a couple of 80's G12t75's, in an X pattern which sounds great with greenbacks and a Marshall also.
So--With the Tc-50 at band volumes, through my Vintage Modern 4x12, I felt like both of these configurations sucked. Notice all the official TC-50 vids are done through V30's? To make a long story short, through lots of experimentation, I ended up doing the Friedman Steve Stephens cab configuration of putting Greenbacks on top and v30's on the bottom, and it rocks like crazy. Since Greenbacks aren't very efficient speakers, but they have such a great upper mid sizzle, it'almost like they're putting tweeters on the top, and they produce they're own bass from just being pushed. The efficient V30's on the bottom produce the balls from being **** near twice as loud with lower mids which add a grind and just basically ground the whole thing. The overall effect is that the cab kicks serious *** while sounding pretty smooth and professional too. Cleans, crunch and all out distortion sound great.
The thing about the Greenbacks is that you're going to get a vintage vibe out of them no matter what, and at loud volumes, i just felt that the TC-50's compression didn't work well with them in as the sole speaker, but paired this way with the V30's, it's like have a vintage vibe on mega-steroids. Think setting the Tc on Red, and getting Black Sabbath meets Voivod! (Also depending on guitar and Pups) Luckily enough, this cab configuration works just as well with my Marshall, so I'm pretty psyched about it. In other words, the Greenbacks do add another dimension to the V30's that really works with the tc-50 to my ears, but I think even though I'm no huge V30 fan to begin with, I think it's good to approach this amp from that speaker, since it was obviously designed around it. Kinda live Marshall JVM's are obviously designed around G12t'75's
Another point I need to stress is that Chinese V30's suck. I tried this configuration with Chinese made V30's initially, and I was horrified at how bad they sounded. I didn't have the patience to break them in so I bought two Mesa branded v30's from the Mesa Hollywood store, and they sound amazing out of the box. They are well worth the slightly extra cost.