The cost/performance tradeoff on vintage preamps has never struck me as that great. The better ones seem way out of line (ARC) and the cheap ones aren't that great. There were some common names like Yamaha and such, but the better ones don't seem to show up as often as one might like.
I'm working on a PAT-4 right now and have a few mods that really clean it up. The problem with any Dynaco preamps is that the controls, jacks and construction aren't very good to begin with. The PAT-4 is also annoying to work on because you have to desolder all the wires to get the boards out. At their age, you have to replace every single resistor and e-cap in them to have a hope of accuracy, particularly RIAA. You could easily put 8-20 or more hours into tuning up an old PAT-4. I'd never pay much for old Dynaco stuff, but I seem to be in the minority. PM me if you want a prelim copy of my mods.
The Crown IC-150A has a terrible reputation for SQ, but it's built well and uses decent parts. You have to like that retro look with chrome. I think it could be easily upgraded to much better performance. I've had a couple but foolishly sold them. The IC-150A used a very failure prone Raytheon voltage regulator, but that's easily dealt with. The IC-150 (no A) seems to be held in a bit higher esteem for SQ, but I've no experience with them. The ones I bought were inexpensive, but I've no idea what the going rate is today.
Probably rarer than hens teeth, but sonically decent, is the Ace Audio preamp from the '70s. It was minimalist, especially the "zero distortion" model, which just eliminated the buffer stage. The RIAA section was a 2-transistor affair very similar to the Dynaco. At that point it was mostly a selector switch and a volume pot in a box! I have schematics for that somewhere if anybody want's 'em.
If one has any DIY skills, you can build a 1st rate preamp for very little. I'd start with the app note on LME opamps from National/TI. I use something similar in my system and it has very few flaws.
