Ok, I've gotten lots of good input from people and I'm very happy and I think I may be getting closer to making a choice. I've heard the 7506 in a store, hooked up to a crappy discman and playing music I wasn't exactly familiar with. So I can't judge them too harshly, but my general opinion was that the sound itself was good, however, the sound stage was very small, blobs in my head I think is the going expresion. They felt/sounded a little stuffy too, but maybe replacement pads would help. I suspect that plugging them into my sound card (DMX 6-Fire) would aid the sound stage problems a bit just by virture of its ability to open things up and place sounds. Also, they don't seem to have any need of a dedicated amp, which is a big plus.
My questions are these; Will a better source make a big difference for them? What's the difference between V6 and 7506? (The only one I've heard that wasn't subjective is that the 7506 has gold connectors and the V6 doesn't. Is this true?) And finaly, what can anyone tell me about the 7509? It seems like a better version of the V6/7506, but I haven't heard anything about it. If it still doesn't really need an amp it's comfortably within my price range, so I'd be eager to learn more.
I would also like to thank warmly everyone who has helped me so far. I know I'm a noob to heapdhones, and this forum, and poeple have been very kind and knowledgeable. I really like this place.
Actually, DarkWolf, the MDR-7509 is the so-called "professional" version of the MDR-V900. As such, the V900/7509 suffers from somewhat boomy mid-bass and lower mids (with precious little deep-bass extension), peaky mid-treble, and (artificially) ringing and echoing noises when compared to the V6/7506. Not worth the $$$, IMHO.
And the V900/7509 is overly efficient (107 dB @ 1 mW) and overly sensitive (123 dB @ 1 Vrms) for such a large and heavy pair of headphones, due to the relatively poor damping of their 50mm drivers. I think that Sony could have improved the damping factor of those drivers by using a relatively high impedance (e.g. 120 ohms, instead of the 24 ohms that the V900/7509 is currently rated at) or by using a lower efficiency rating (e.g. 92 dB @ 1 mW instead of 107 dB @ 1 mW).