some worry
With Class A/B designs which pull variably off the rails in response to the signal, there is some worry that without a seperate supply a high draw from one channel will cause the other channel to run less optimaly. There is also crosstalk with some signal appearing on the other channel.
However, Class A amps like the A75 pull steady current off the supply so a supply designed properly to take current from both channels should have none of the first problem described. As to crosstalk, yes, monoblocks will give you better performance, but how much?, that may not even been measurable by you.
I read that one man disliked monoblocks as he felt the lack of mechanical connection between the two channels made them slighty different and they sounded "wrong" to him..
Monoblocks are a great way of dealing with the heat, and in my opinion they look darn cool next to each speaker with some big cable.