NAD 3020A Integrated Amp

The speakers are probably revealing the flaws of the NAD. The NAD has a dry sound, and also lacks transparency. I would try a better amp myself. I think people liked them because they didn't have the solid state harshness, that a lot of other budget amps had in it's day. I'm not trying to be rude, but I think the rest of the system could do better with a different amp. I'm not familiar with that Sansui either.

I didn't try the preamp out when I had it (3020), but its worth trying, if it has it. Sometime they sound almost as good as a dedicated preamp. People do it with receivers to, and report good results at times. I've done it with others(brands/models) in the past, and received nice results. It depends how well it mates up to the particular amp you might try, just like a stand alone preamp. A lot of them put the feature on in case you want to go to a bigger amp in the future, but sure doesn't limit you to the quality either. I was satisfied with one setup in the bedroom for awhile, that I used the full integrated for TV, the pre-out for a tube amp for music. That worked fairly good, until I traded, and moved on. Part of the fun of the hobby.

If you still have those 4312a speakers, see if you could get a hold of a Mcintosh 2100 amplifier, at least to try the combo. That amp, and those speakers sound like they were made for each other.