ATH-A700 vs. ATH-M50

The M50 is by far more durable. It's a studio/dj style headphone, so it's built tough. The A700 is... less so.

Bigger drivers generally lead to a better bass presence, but within the realm of headphones this is fuzzy at best.

It's hard to classify one as "better" than the other. Heavier genres with more bass will enjoy the punchiness of the M50. Jazz/vocal/classical sound much better from the A700. Gaming also would prefer the A700 for the bigger soundstage.

Depends what you mean by "quality", and your choice of headphones in that price range is huge. In the studio line, the Shure SRH 840 and M-Audio Studiophile Q40 are all comparable. In terms of bass, M50 has the most punch, Q40 has the best rumble and soundstage. I also have a personal preference for the Phiaton MS 400. It's a very coloured sound, but lots of bass presence and a fun sound signature.

You're going to have to be much more specific in your requests if you want any specific recommendations.

I say the M50's and SRH840's are solid built. But, with different sound signatures. 

    I can only comment on the ATH-M50, and I will say they are extremely durable. I have fell asleep on top of them at least ten times already and I weigh 160 pounds. Not to mention I once dropped them down an entire flight of stairs and both the M50's and my iPod, which went along for the ride, where fine. I would not recommend that you do any of the things that I did, I am only saying that if you were to do such a thing the M50's would have a pretty good chance of surviving. 

    As far as sound quality is concerned I would describe their headphones as being fun to listen to. The punchiness of the bass gives the headphones life, while the highs are very crisp and clear without any harshness. The mids are there, but slightly recessed IMO. These headphones will do well (not the best) with almost any genre, except classical perhaps.

There are amazing deals on the the sennheiser 555 right now. It's better for music than the AD700 and I prefer it for gaming due to it having more bass presence. The AD700 is severely lacking in the low end.

I would recommend the M-Audio Q40 then. Much more neutral sound for mixing purposes, and with a bass which I feel is more suited to your genres.

If you want punch, the M50 is more suitable. The Q40 doesn't hit quite as hard, but has a deeper and richer bass (especially when amped). The Shure SRH 840 is also a popular contender in this category. Probably the least bass of the three, but the best mids. All three are studio cans, so are reasonably durable.

I've been writing a comparison/review (still incomplete) here: http://www.head-fi.org/forum/thread/512762/review-shure-srh-840-audio-technica-ath-m50-m-audio-studiophile-q40-pioneer-hdj-2000

I have not heard the XD-53, but it reviews quite well. Once you're looking into DJ style cans though, your options open up quite a bit. Here's a good comparison/review of many models: http://www.skratchworx.com/reviews/headphones_2010.php
As a general rule, DJ cans will be more durable.

The M50 in my experience benefits very little from an amp. I can't say for the XD-53, although going by the stats I doubt it.