Well, the 101b was voiced around V30's. That said, nothing wrong with greenbacks...they just don't have the same low end grunt and are looser. As Nico said, they're great for plexi mode, but I prefer V30's for high gain. YMMV.
greenbacks go great with the xtc imo. adds some good crunch and livens the amp up considerably versus V30s. brings on a bit more of a marshellesque flavor to an amp that is otherwise on the dark side..
I would say a rootsy, raw, classic sound, in my opinion. Already mentioned, brightens up a "blankety" amp. That's what I thought when I played mine with greenback/creambacks.
I use V30's and serves me good. But, if I had the means, 1 of each cab.
I have cabs with Scumback M75's and nicely broken in V-30's for my 101b Classic.
Compared to the V-30's the M75's are a bit darker, and have a much faster breakup. Nice and crunchy like a greanback should be!!! I prefer these speakers when using the amp by itself. In a live band setting the V-30's cut through better.
People discount GB's for harder amps like the 101B, when in fact they are perfect for stage. You may not get all the bottom end in the home, but you don't need it playing live. I like GB 25 watt speakers with almost everything (no H30's though).