I expect that there are differences that would be more evident with some designs than it is with others. I've heard good amplifiers that were monobloc and others that were on one chassis.
I own a pair of very expensive monoblocs and a fairly expensive single chassis amp. My favorite is the single chassis amp, but, I have no doubt that this is the result of all of the other much more fundamental differences between the two--pushpull vs. single ended, 2a3 tubes vs. 349, different input tube, different everything else. The builder of the single chassis amp could have easily built the amp as monobloc's (each channel and the power supply are entirely separate modules) and chose not to so do even though the additional cost would be relatively small (the additional power supply module would not be that expensive).
For me, single chassis vs. monobloc would never be close to a deciding factor.
