Hi,
I own all of these cans, so I can tell you a little about them driven by a Soundblaster X-Fi Titanium, Corda Cantate and Symphony (btw. both Cordas do not have such huge differences in presenting their sound, as it is always said).
All of these Sennheisers have a pretty narrow soundstage compared to the big Beyerdynamics or AKGs.
The HD555 and HD595 can easily be driven by a sound card. The HD600 and HD650 require an amp or a very good headphone jack.
This is how I'd characterize the sound:
HD555:
Quite undefined strong bass, somehow cut highs, but the mids are fairly well presented. Sound does only improve a very very little bit by amping it. Compared to the following headphones, these are very mouldy sounding ones.
HD595 (150 ohms version, old model):
Best buy for an unamped system. Clear and well balanced sound image with a nice deep bass and very tolerable highs and -like the HD555- well presented mids, when connected to the X-Fi. Using an amp makes them to me intolerably shrill and adds a taste of hollowness, which I do not like. Others might say they sound more 3-dimensional using an amp. Well, I don't like it.
HD600 (with black membranes, old model):
These ones are ideal for laid-back chamber music listening and also good for some kinds of rock music. Yes, they are laid-back sounding, but somehow involving and direct, too. To me, this is very special. The mids of these cans are their strength, the highs are nice (rhyme!
), not aggressive or harsh, but the bass is some kind of weak and tends sometimes simply to drown. They sound clearer and more airy than the HD595 and can profit by an amp with 0 ohms output impedance.
HD650 (with silver membranes):
Basically they sound like a darker HD650, with nearly the same presentation of highs (which are nice) and darker mids. The bass is compared to the HD600 very lush and to me in many situations preferable. But beware! To some people these headphones are so laid-back sounding that they say they hear a light curtain all over the sound.
The HD555 is the most comfortable of these headphones. The HD595 is nearly as comfortable, while the HD600 and HD650 are tighter and less comfortable, especially when new and your head has a size above average.
I really can not say a clear recommendation for any of these headphones. All of them -except the HD555- are soundwise pretty enjoyable cans to my mind.
And remember that the spread for standard factory models is also a matter here. So some people -I think- would partially disagree with my experiences because their model of the same Sennheiser just sounds a little bit different.
I hope I could help somehow.