There is still one of my original questions that nobody has addressed yet:

Would a 518 make a "significant" improvement to my sound?

I've always wanted one of those, but about 25 years ago I brought one home as a demo to play with, and honestly couldn't hear much difference. But I was running analog amps and speakers (with a 565 as the preamp).

I'm wondering how much difference it would make in the performance of a pair of DSP5000s.

Theater: 565/562v, 519, Analog speakers (NHT & Velodyne) Pioneer LaserDisc, Sony PS3, Comcast Cable
Office: 561, DSP5000.2, NHT rears, Acurus Amp, Technics Turntable, Sony SACD Changer, PC Digital out