I would say out of the vintage Stax that I've owned, the SR-5NB (gold edition) are way ahead of the SR-3, and can beat an HD600 or HD650 for $200 including energizer. They have good bass, warm mids, and sparkly treble, without funny resonances or peaks, and seem very accurate. I had Gamma Pros and thought they were great, but they seemed a little rolled off at the low and high end of the frequency spectrum, and the SR-5NB (gold edition) have more extension (maybe because of the ear-cups design or something).
I actually have a set for myself and a set for my son, and each time I was able to do it for under $200, although I have seen the SR-5NB go for $200 without energizer here and on eBay, but it should still be doable for under $250. This sounds better to me than a stock SR-001 Mk2 which is the same price, and is more comfortable to boot.
As another comparison of the sound, when the SR-5NB and SR-003 are both driven by my SRM-1 MK2 (high and normal bias), they sound VERY similar to each other, which is to say great. The only way to get a SR-001 to sound that good is to do the "AudioCats" mod to one, which could cost $80 in parts and about 4-6 hours in labor - I have a stock one and modded one, so I know it sounds very close after the mods.