I found this comparison:
"Here you go! Some acoustic plus vocals then some death metal guitar, the first version of each is the 8i6 and the second the Bluetube, in solid state mode.
The acoustic and vocals were recorded with Studio Projects B1's, guitar first with both mic's tracking at the same time (and right next to each other) and then vocals, again both at the same time and with the mic's right next to each other. The electric guitar was done with a SansAmp GT2 run through a splitter pedal to track both inputs at the same time, so in both cases the source should be identical.
I used no hardware or plugins to manipulate the sound in any way- no compression, EQ or anything else, totally vanilla!"
It's not 2i4 vs 22vsl, but I'm pretty sure these are the same preamps you'll find in the 2i4 and 22vsl respectively. Just don't quote me on that.
Listening through a very poor setup here, but I can readily tell that the 8i6 sounds a lot better. Not even subtle. Still, I'd like to hear what you guys think.