I have never compared the different amp channels. I can tell you that I do think the front channels sound a bit more transparent than with my old 20 year old ATi 1505, but that can be wishful thinking.
VTV doesn't make a 5 channel Purifi amp, neither does Buckeye. I thought about doing a 3-channel amp and a 2-channel amp, but ultimately wanted one-chassis and didn't think I needed to spend extra on Purifi for the surrounds.