looking at Grado SR325's, Senn HD580's or AKG 271's. They seem to be in the same price range, give or take 50$. I don't have a headphone amp, but have this for playback: Sony CDR W33>Adcom GDA 600>Adcom GSP565>headphones???. Iv'e been using a 20 year old pair of AKG 141's and I think the drivers crapped out. Then I bought a pair of Sony's at a local Circuit City, loud but zero detail. Thanks.
You're going to definitely need a headphone amp for the senn's, the cans themselves are relatively cheap, you can find them new on ebay for $150 iirc. The senn 580/600/650's are scalable in the sense that they will continuously sound better with different types of amps (in terms of synergy, solid state vs tubes and so on) and aftermarket cables. I've never those Grado's nor the akg's but you should check out head-fi.org for reviews on them, I'm sure there's plenty info available on them. As for the bass response on the akg 501's...its often considered on the thin side by many head-fiers and I personally find them to sound a bit metallic sounding. The senn's have a more organic, big speaker-like sound to them that makes them very pleasing to the ear...especially with tube gear. You don't have to break the bank on an amp, just check out head-fi for an amp that may meet your budget guidelines.
The senn's are also very comfy for long-term listening compared to Grado's which typically put my ears on fire after 30 mins of listening with even the comfy pads. Plus, the bloody cord can be a bitch to deal with (its thick and a bit cumbersome).