I had a 372 as well, though I didn't get around to experimenting with the Pre/amp connection. New they went for under $1000, and I'd say it was decent mid-fi. If you want an improvement, I'd look for a used 375. If you really want to go for separates, Parasound makes some very nice sounding stuff, check out their 2100 or P3 pre amps. Or you could stay in the NAD family with the 165 pre, and an amp from either company would be fine.
These items come up for sale on the used market, but you have to be patient.
These items come up for sale on the used market, but you have to be patient.