My thoughts, from a fair bit of experience on a 101B (mostly at half power, mostly through V30s), some experience on a 3534 (through Creamback 65s), and remembering my experience playing a 20th XTC for a short while (through V30s).
Blue channel on the 3534 is great IMO but different than 101B. Blue 3534 to me is still thick and has a similar fatness as Blue 101B, but 3534 Blue to me is more a growl sort of tone than Blue 101B, which is more of a purr. A LOUD purr.
Blue 3534 is more open; Blue 101B is more compressed. The range of gain on 3534 Blue is very flexible, from 70s AC/DC sort of barely dirty to 80s crunch, with that Alice In Chains sort of low-mids growl. It reminds me of 20th XTC Blue but better, not as congested. 3534 Blue to me is a better sound for rock rhythms; 101B Blue is better for leads, especially that classic liquid lead sound.
101B Red to me is compressed, dark, and crunchy; the post-2004 version very compressed. It can cut if you dial it in right; the Treble pot is log taper, so you have to turn it up well past noon to get it like turning a 2204 Treble past 8 o'clock.. 20th XTC Red IMO was much more open, less compressed, more cut; a bit saggy on Vintage Structure, great for high-gain rock tones, and tighter on Modern Structure, great hard rock or metal tones. 3534 Red is not as dark as 101B Red but has way (way) more gain. At 9 o'clock it did a great tight chunky EVH, at 10 o'clock it did thrash; above 10 o'clock for me it was unusable. With the C81 cap clipped gain mod that's on TPG, the gain range IMO shifted by about one clock hour position, but the tone is a hair less tight; it did GNR squishy crunch at 9 o'clock, EVH at 10 o'clock but not as tight as without the mod, and above 11 o'clock was unusable. It had nice low-mids and decent cut, but to me sounds darker and less cut than 20th Red and way less flexible than 20th Red.
So for me, for Blue, 3534 and 101B are a tie, with 3534 winning for rhythm and 101B winning for leads, and 20th Blue in distant second place. For Red, 20th for me is first place, then 101B--it's dark but has better range of gain and useability--and 3534 by a hair is last, not as dark as 101B but much less flexible or usable. Big design mistake IMO that they didn't give 3534 Red a range of gain that starts lower, like 3534 Blue.
