The SR6500 are very very good set of comps. I considered the polk 6500 on the higher-end. Why do you think you are going to be able to hear a major difference through your speakers by how high up the food chain in the HU department you go.
If you are looking for a great SQ deck then your generally looking for these things no matter who makes it:
24 Bit DAC--Burr Brown being the better IMO
4 volt or higher pre-out voltage
EQ ability
Quality of internals
X-over ability if going active
Here are some HU for to look at that are considered many to be on top of the SQ heap. This is not all just a short list off the top of my head. Just compare them to either Nakamichi you mentioned to help you sort out the Naks. I think most would consider the CD700 equal to or be better than some of the deck listed.
Alpine 7909, 9835, 9855
Eclipse 8053, 8443, 8454, 8455
Clarion DRZ9255
RockFord (Denford) RFX8250
A bunch of the Denon HU
If you are looking for all SQ then look into a mid to higher end amp, this is personnel preference though. However, there are some very good budget amps out there that will shock you.