to me - a big part of the seperates arguement is that amplifiers have great longevity. You can look at an AVR from fifteen years ago and the cutting edge might have included composite video. Today,that's worth pennies on the dollar to most people.
On the other hand, a fifteen year old solid amp is, to me at least, still worth a great deal. Only the processing bits need to be upgraded for htmi 1.5, 1.6, 1.7 etc...
And although axioms are very efficient, this is an evolving hobby for many. Perhaps you'll someday want to ply with a pair of power hungry speakers... i.e thiel, etc. today's "seperate" investment will likely continue to hold it's own in many future senarios
