If you know this has been discussed to death a thousand times, then why not use the search function to find those discussions? I hate to sound like a jack***, but us older members really do not enjoy regurgitating the same information 10,000 times in a row.
But, I'm being confrontational. There's a time for that, but this isn't it, so I'll be helpful.
The A900 sounds very different from the HD595, but in absolute sonic performance they're about on par. The HD595 is relatively well balanced, but does tend to roll off at the frequency extremes. Upper treble is a bit soft, and deep bass is very lacking. What's there is nice and tight and pretty slamming, but it's not a basshead phone by any means. The mids are warm, the soundstage is definitely not bad, and the overall sound is fairly forward, not laid-back as it can be on some underdriven Senns. The sound does tend to be a bit grainy and overall resolution is not bad but could definitely be better. It's this lack of fluidity and grain that I really don't like about the HD595, so pair it with a liquid-sounding amp (check the amp forums).
The A900, on the other hand, has very good performance at the frequency extremes. Well, good for the price, anyway. The bass is powerful, deep, and could be tighter, but certainly not bad in terms of control. The treble is extended and pretty detailed, though the lower treble is emphasized and a bit steely. The midrange has issues though; there's a bump in the upper mids that screws up midrange tone somewhat. You're not going to get the same sense of warmth with the A900 as it's definitely a cold(er) sounding phone. But, at the same time, that elevation in the upper mids tends to give the headphone an artificially large soundstage, which works quite well with electronic music.
Honestly, it comes down to practicality, and which sound signature you prefer. Do you want an open or closed phone? Do you value nice warm mids over solid bass and detailed treble? Because in objective terms, they're both about on par, and both not bad, but nowhere near the top tier.
If you have an amp already, check out the HD600, it's in another league. But, if you don't have an amp, you will have to include it in the price, since the HD600 will not give you good performance for the price unamped. 300 ohms is way too much for a soundcard or mp3 player to handle.
If you want the maximum sound quality for these kinds of prices, check out the Stax SR-001 system, or the SR-005a - maybe even the 2050 if you stretch the budget a bit. A used K340 with a used powerful tube amp to drive it will also work well, but with used headphones and K340s especially, you have to make sure that your pair sounds good and is in good condition, which isn't always the case. So, it's a risky buy, but if you get a good one, you'll have a high-end headphone for a mid-fi price.
