I heard both SR-507 and SR-007A on a very good modded Stax tube amp - for my personal preferences I preferred the SR507 although both were great. For my tastes they are actually the best phones I have heard, I prefer them not just to the 007 but the LCD-2, HE6, T1 and certainly the disapointingly bad (for my tatses) HD800.
This just highlights a point about asking advice - all you're going to get is honest opinion. You have no way of knowing if your tastes are closer to the guy who thinks the 507 is worse than all those phones, or me who prefers them.
One thing I'll say is you can get the SR407 (or probably even the SR307), buy some replacement SR507 pads and you'll basically have a much cheaper SR507.
Unfortunately I have not heard the SR-007, but I agree with you on the rest. I prefer the SR-507 over the HD800, Audeze LCD2, LCD3, Hifiman HE-6, HE-500. Actually I even preferred the well broken-in pair of SR-307 + 323 of the Stax dealer over the HD800.
I really wanted to buy the HD800, for its comfort, engineering, looks, made in EU, etc. But the sound was a bit artificial when compared to the Stax, and I always felt something lacking, and conversely, I was always surprised by how good the Stax were. Too bad the 507 were a bit too big for my head, and are pretty ugly anyway. I didn't have access to exotic amps often quoted with the HD800, so take my opinion with a grain of salt.
I spent some time with the HE6 and HE500 too. Not bad at first, but after a while it became irritant that the Hifimans had a slight zingy quality to their sound. I tried to switch the earpads, with good results, e.g. Stax 007 pads improved them considerably, and I read the Audeze pads may improve them even more. However, they still sounded a bit zingy. Removed the pads, and it became clear how they could potentially sound: very, very deep (much deeper than with pads), with a bit of bass boost, and prominently zingy treble. Not bad at all, but the others were more accurate. I could say a similar opinion about the Grados, which have a similar, but even more accentuated sound signature. I hope Hifiman could solve these issues and make a ~500 EUR headphone which is accurate enough.
As for the Audeze, they did sound a bit dark at first, but I got completely accustomed with them in a short while, and they still revealed true music - with some lack of treble, which made me feel older than I am :). I would have been happy with either Audeze, if I didn't hear the Stax 507 with the amp I've got. A lot of instruments (fagots, trumpets, violins, percussion, piano) sounded more natural, more extended and with more resolved body resonance on the SR-507, not mentioning the ambience of the recording (e.g. on Jazz at the Pawnshop, Vol.1.). The SR-507 is similarly open sounding, but "calmer" (could I say darker?) than the HD800, which screamed out every detail on the recording a bit too prominently (I tried both early and recent 16xxx serial number samples).
Unfortunately I could not get to the Fostex TH-900, it would have been nice to try. But since the 507 sounded so much better to me than the other, highly acclaimed headphones, I think the only headphone I would upgrade to would be the Stax SR-009, at several times the price of my current setup. So this SR-507 is good enough for me, and I am surprised by joy every time I listen to it - in the evenings, since otherwise I mainly listen to speakers for more realistic image depth :).
Edited by zolkis - 8/22/12 at 11:52am