Changes to older system

The easy and cheap stuff…

Overall I still enjoy the sound, but I’ve always thought it was bright and bass shy.

One could add a subwoofer.

A DSP would be a way to alter the frequency response.
Or something like a TDAI 3400 which can do mostly the <500 Hz part.

First and most obvious choice is cabling, but what?

It is easier for a cable to alter the bass to be less.

If the speakers have a rear port they could be moved closer to the wall. And depending on their pattern, one may be able to toe them out a bit??

I’d throw these in, they’ll make the difference. I’d also find some thick OCC speaker cable, that will make a change for the better as well.

I’m a NAD fan too, always a great sound.

I would start with the preamp. I have been a long time appreciator of NAD and have owned a couple.

I think a tubed preamp could have a big impact / improvement on your system. In an attempt to recommend something cost effective… probably Rouge. A tubed preamp for sure.. an older Audio Research, VAC, or Conrad Johnson all sound like they would be really complementary.

Do you solder? I've been experimenting a lot with copper, cables, re-purposing and various sounds on mid-range equipment, much of it NAD too. A few days ago I made some jumper interconnects (pre-out/main-in) from 12AWG 240v rated Home Depot ROMEX and was very surprised with the results, a deep defined bass with light airy highs. I'd probably get beaten up by this post, but if you solder, it would be a $20 experiment which might surprise you and bring a completely new dimension to the equipment. It's cheap by the foot, and I'd give it a shot with the speakers too - it's a lot of copper (albeit not refined like OFC) but gives a maximum throughput.