Well, I've been using the DT990 Pro (250ohm) w/ Xonar DX and Fiio E9 for several months now and I've been quite happy with this setup. The only real drawback I've found is that with the default settings on the Xonar software mixer, the mids are quite recessed and the treble was downright annoying. Spending about 30 minutes in the Xonar mixer control panel tweaking settings helped me to alleviate about 98% of this. On the Astro mixamp, I didn't seem to notice this effect nearly as much but, there's not much you can do to change the sound there. Regardless, these are still a MAJOR step up from the A40's. These headphones are going to be a bit bass and treble-forward no matter what you do but, I knew that going in and after my first couple hours using this setup, I stopped noticing it. My gaming habits are probably 70% "immersive" gaming and 30% competitive and IMO, they work near-perfectly for this purpose. The first time I heard the sonic boom of a Hyperion orbital drop in Borderlands 2, I actually thought someone was dropping heavy machinery outside my house until I actually saw the object in-game. Playing dead space with these headphones has been a terrifyingly awesome experience. When your character hears those random, creepy whispers, it actually sounds like someone just whispered right in your ear. Positional audio has been solid (granted not as great as some other cans might be) and aside from players using some form of silence enhancement, I've yet to have someone sneak up behind me in BF3 or CoD without me hearing them first. Naturally, watching blu-ray movies (especially action and sci-fi films) is an immersive experience as well.
Music-wise, being a warmer, more bass-heavy setup, dubstep and DnB sounds amazing. I would even go as far to say that these headphone more than hold their own in Trance and even rock music. They're at least head and shoulders above what I used to use.
Overall yes, I'm quite happy with my setup - especially considering what I paid for it. Honestly, spending 10 minutes on google shopping/ebay/amazon and having priceblink or a similar plugin could probably get this setup for around $350-ish with zero hassle. (And I would still be happy paying that price). Some patience and low-balling ebay sellers will only drive that price lower. I can't really think of a way to get more "bang for the buck" for my purposes.
In answer to your second question, assuming you're referring to just the ohm rating (impedance) of the different headphones, that basic gist is that higher-impedance headphones require more voltage to drive but, are more "tolerant" of higer-output impedance amplifiers and thus, less likely to experience distortion if you're not using an expensive, low output impedance amplifier. Lower impedance headphones require less voltage to drive and are much more efficient but, require higher quality (and generally more expensive) low output impedance amps in order to drive them properly without distortion. Wikipedia has a pretty good explanation here: http://en.wikipedia.org/wiki/Headphones#Impedance and I'm sure people on here that are more educated than me could explain it better.
In practice, what I've found is that the DT990's can be driven straight off of my Xonar or from the mixamp but, they are quiet and a bit muddy. With a ~$100 FiiO E9 driving them, they sound amazing.
Hope that helps and thanks again to everyone who posted to give advice!
