Thanks for the welcome. This is exactly the kind of help I was seeking.
To answer your questions:
Do you want to listen to music in 2.1/5.1 configurations? It depends. If I'm digging deep into some 180 gram vinyl I would use 2.1, but if I'm reading the paper or setting my digital collection to random, 5.1.
When you fed the pre-outs from your Denon to the McIntosh integrated which inputs did you plugs them into? This is where I went wrong, and I fixed it last night. I had the preouts from the Denon going into the Tuner input. I changed that to the tape in on the ma6500 and then the tape out on the 6500 ran to the Denon's CD input. This allowed me to use the ma6500 to drive the front L&R channels and still use the Denon's level setup.
Why not into the 275 amp? That may well be what I end up doing. But I love the look of the ma6500 and I'm guessing it has a higher build quality for some of my analog components.
I think your suggestion makes perfect sense. I did make a speaker switch so that I could use the front L,R and sub channels with either system.

I'm just not sure which method is best. My current plan is very complicated. I tend to agree with you, simpler is almost always better. But what do you think of this:
SOURCES-
Computer to DACMagic
CD and BRDP Digital to Denon
CD and DACMagic Analog to ma6500
OUTS AND INS-
Denon Front Left and Right Preout to ma6500 Tape Input
ma6500 Tape Out to Denon CD Input
ma6500 Main Out to mc275 unbalanced inputs

How else could I improve sound while preserving ease of use?