You are where I was a while ago it looks like your system is pretty well matched now. Yes buying a pre would help but the pre has to be better than the NAD. I had a Audio Reasearch SP16 tube pre with a HT bypass I used that instead of my denon for 2 channel and put it on bypass for HT and suddenly it gave my system a warm smooth tube sound with more sound, I didn't realise how much the solid state receiver was cutting out. Will it make a difference you will just have to try.
I keep on looking at your system to see where $$$ could help and you are definally at a point where you need to break out of the solid state best buy componment. But there are several areas you can start the pre is not a bad start. I don't think byamping out of a receiver or 2 different amps is a grate idear. You are better off with one good amp. May also want to play with cables/wires a bit I didn't see what you were using.
I keep on looking at your system to see where $$$ could help and you are definally at a point where you need to break out of the solid state best buy componment. But there are several areas you can start the pre is not a bad start. I don't think byamping out of a receiver or 2 different amps is a grate idear. You are better off with one good amp. May also want to play with cables/wires a bit I didn't see what you were using.