A900:
Pros - Good soundstage, great frontal imaging, which most headphone without angled drivers struggles with (yes, A900 has angled drivers). Very good sound imaging. Sound signature itself is of a very balanced nature. Very smooth from the bottom all the way to the top. Nothing is extremely forward than others. It's a great all-around headphone for all genres.
Extremely easy to drive, it is commonly acknowledged as the best performing, unamped closed headphone for under $200.
Cons - Jack of all trades, but master of none. If you listen to a specific genre more than any others, you may find a headphone in the same price range that's better. For example, Grado SR-225 for rock, HD580 for classical, so on, so forth. However, none really performs decidedly better without an amp though.
Isolation is good, but not as good as some closed headphones that are designed to be portable. It is definitely a large headphone designed for home use.
Sennheiser HD580/600/650:
Pros - Open, airy soundstage, very natural and soothing sound. Very good for classical and jazz. Very comfortable open-air design with very little heat build-up. Some stretching will relieve the "clamp" feel over time. HD580 and HD600 are arguable in the same class as A900, but HD650 is definitely a step above and beyond.
Cons - Difficult to drive, you just can't get anywhere near their full performance without an amp. Out of weaker CD players you can't even crank them up to an enjoyable volume. They can be considered too laid back for rock music. Soundstage extends only horizontally, frontal imaging is a bit lacking. Sound imaging is less precise in comparison to A900.
Ultrasone HFI-650/700:
I haven't heard HFI-650 myself, but I've heard the 700... I assume their basic sound signature is the same (as noted by other head-fi members), and the main difference is in the strength of the bass.
Pros - Very powerful bass, one of the most powerful bass out there in any headphone. I suppose DT770 has even more, but Ultrasone headphones are known to have better balance in its sound with its powerful bass, and DT770 are known more as just a basshead headphone. Just as easy to drive as the A900. More portable design, could be folded in two different ways, and has more isolation than the A900.
Cons - Ugly headband, a bit more uncomfortable compared to either Senns or A-T headphones. Although sound signature is balanced and powerful, it also sounds a bit unrefined.. there's some harshness in both the bass and trebles. Soundstage is smaller, and frontal sound imaging lacking a bit as well.
