You won't see any benefits using an amplifier or receiver with the A900 (40ohm) and AD900 (35ohm). The Koss KC75's (60ohm) would benefit more from an amp. Both cans are easily driven and would work fantastic out of the Prelude or any other X-Fi. Normal X-Fi cards will benefit greatly with upgraded op amps and a better power filter capacitor. The Prelude also has a more powerful output section than the regular X-Fi's so will drive headphones even more efficiently. The Prelude has some of the best DAC's and op amps in the industry and is comparable to using audio gear worth thousands of dollars. I'm not kidding, go over to head-fi.org and you'll see what I am talking about. The same goes for regular X-Fi's with upgraded op amps (op amps are a top component when talking about sound and the stock X-Fi op amps should burn in hell) but the Prelude is that much better. The DAC's found in regular X-Fi's are actually quite good quality and that is why the X-Fi with modded op amps is popular around the audiophile world.
The ability to digitally playback files which have been ripped in lossless format is like having a $$$$ CD player. Throw on top of that a quality external DAC which costs $$$$ to match the Prelude and your looking at a few paychecks. PC as source for audio is unmatched for price/performance and flexibility.
ANYWAYS back to the headphones. Because of the open air design of the AD900 it offers some noticeable gains in sound quality opposed to the closed design of the A900. I'll try to explain my best between the two.
Bass - This is the one area where the A900 is superior in terms of overall amount of bass. It will be more like the subwoofer in your car that goes really low. It will have more of an effect of covering your head in bass opposed to the AD900. The AD900 has less overall bass but what is there is more defined, impactful and tight. However this is not to say it is lean on bass. It has a neutral bass compared to all headphones so would run somewhere in the middle. It all depends on the type of bass you want. If you want real thumping bass then the A900 is probably better for you. If you want clean, fast and tight bass then go for the AD900. REMEMBER some will like one type and some will like the other and this is only a guideline.
Mids - AD900 is a mid monster. You like vocals? Mids, mids mids. You like Maynard James Keenan doing what he does best? Mids, mids mids. The AD900 really shines here whereas by comparison the A900 lacks. It's not so much that the A900 doesn't have mids, it's more of the fact that it has stronger bass and treble which seem to drown out the mids.
Treble - Both are probably a winner here but I tip my hat off to the AD900 because they aren't as overpowering and are more neutral with the mids and bass. The A900 might be able to reach a bit higher but you'll find yourself taking your headphones of quicker for a break.
Comfort - Supposedly some of the most comfortable around. Both use the same system and should be equally comfortable.
The AD900 sounds like the better overall can for a wide variety of music. The bass is still good but definitely not overwhelming like the A900. If you need large amounts of bass then the A900 will be a better can for you. The mids are much better and the treble is more natural on the AD900. I'd say the Koss KC75 and AD900 are probably about equal in terms of bass but the AD900 will have better detailed bass. For gaming both are very good because of their excellent sound stage and imaging. These cans are among the top suggested for gaming so you'll be very happy with both in this regard. The AD900 has better sound stage and imaging but not by much. From reading around the AD900 is worth the extra $40 for improved sound quality so thats the can I went with.
Here's a guy who did a review of the AD700, the AD900's little brother:
http://www.headphones.com.au/forums/viewtopic.php?t=10 This would be a good place to read about the signature of the cans. You'll notice at the end of the review he says the AD900's impressed him even more. Later in the thread in one of his posts he says "The AD900 sounds better than the AD700, that's the difference. Better clarity, detail, separation, balance, smoothness, bass, dynamics etc."
Cheers!
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