Changes to older system

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.

First and most obvious choice is cabling, but what?

Actually, wires are the last thing I’d worry about here, but a lot has changed/improved over the past 30 years so I think a complete overhaul is in order. Start by identifying which speakers really grab you and then I’d recommend finding an integrated amp that has enough power to drive the speakers and is a good sonic match for them. For a source I’d recommend an Innuos Zen Mk 3 that you can load all your CDs into and have all your music at your fingertips, and it’s also an excellent streamer so you can explore worlds of new music — much of it in high res. Only then would I address the cables. Anyway, that would be my plan of attack FWIW, and best of luck

Actually, wires are the last thing I’d worry about

Depends on if new equipment is desired. I used to take an equipment first approach, now to the point of tuning my equipment with cables. Once a non-believer in cables to sound, it's now one of the main things I've found to make a huge difference. Sometimes to the extent it sounds like I've swapped out the component..