how to pick a preamp?

Due to a personal ephanney (was going to build a 4 ch. Stereo) got the 1st amp a parasound hca 1200 set it up with a B&k ref.30 for its capability of 4 ch stereo. After listening to just 2 ch stereo (as I was told, (if not scolded) for a few months I understand now!!! It can't be improved by more chs. So new direction-- going to keep it 2 ch.but now concidering a diff. Pre as I love the sound I get from the hca 1200. How do I evaluate a good pre vs a great pre. Funds are limited to about 3 to 5 hundred for a used quality vintage pre. I want accruate, quiet warm stereo sound. Any help would be appreated!thanks to all those prevouis members who tried to educate me earlyer. It was not in vane just had to here it myself and you were right!!!