DT990 sounds way better than M50 IMO, but does have a more V-shaped frequency response. Sound quality is much higher on DT990 - bass is much more textured, better layered, tighter and more powerful, yet also more controlled than the bass on M50, which could often intrude into the lower mids. The midrange is a bit less present on DT990 vs. M50, but it has tons more detail and is more even sounding as well. The treble is quite hot on the DT990s, but not harsh and of great quality which is really important and means that it can be equalized down pretty easily with great results. The treble on M50, on the other hand, while more neutral and easier on the ears, is also of much lower quality - it has a very noticeable metallic coloration to it, it is quite thin and pretty "dead" overall, lacking real air, depth, proper timbre. DT990 highs are so much more vibrant and alive. Probably the biggest difference between the two though is the stereo soundstage - M50 has nothing on the DT990s in this regard at all. In fact, M50 has no real soundstage to speak of - it is all left and right with almost nothing in between and little to no sense of depth front to back. Plus it has a fuzzy, poorly defined imaging. DT990 has a huge soundstage by comparison with excellent left to right stereo separation and a great sense of depth to the sound. Separation is much better on DT990 and imaging is much sharper than what you get with M50. Really, these two headphones are in completely different leagues - DT990 belongs in a much higher tier.
With that being said, I don't recommend the 32 Ohm version of DT990 as it won't sound quite as good as its 250 and 600 Ohm siblings. Even if all of them will be driven unamped straight from an iPod level source, the higher Ohm versions will still sound cleaner and better because they actually put less strain on the amp than the lower impedance model leading to lower distortion and other unwanted artefacts of headphone/amp interaction. The only downside is that the higher Ohm versions will obviously play quieter, although I would consider the 250 Ohm one just about loud enough for listening to mainstream music through an iPod or source of similar power output. Ideally, you would want the 600 Ohm version for best sound quality though, but 250 Ohm is not far behind and both sound significantly cleaner than 32 Ohms out of stuff like iPod. Also, I recommend DT990 Pro instead of the Premium edition as it costs a lot less and sounds pretty much the same with some minor differences due to the somewhat different housing and clamping force.
EDIT: If you are looking for warm sound, the HD650 is the one you want though, but it also costs more than DT990. With some equalization, DT990 can also be made to sound warmish, but it is really a rather dry, analytical headphone at heart. HD650 will give you a smooth, lush and full presentation that DT990 will not and also probably provides somewhat better overall sound quality as well, which is understandable given the price difference. I really like both personally, although if I could only own one, it would definitely be HD650 as it's the better sounding, balanced and versatile of the two. Both wipe the floor with M50.
As for the lower end Sennheisers, I own the HD555 which I modded slightly by removing the "sound reflectors" from inside the cups behind the driver which improved the sense of space and tightened up the bass a quite a bit. I would say that the HD555, which is apparently almost the same as HD558 and not too far behind HD598 (they all share the same drivers), retains a warm, full and lush presentationthat is quite similar to that of HD650, but lacks the resolution of HD650. The 500 series are quite a step down from the 600 series in resolution and transparency, but are still warm and inviting in presentation. They sound very pleasant and fairly accurate for their price. They are still much more natural sounding than M50 to my ears, especially in the treble, where the open backs usually separate themselves the most from closed backs from my experience. Basically, HD555 is free of the metallic coloration of the M50 and sounds much more delicate and refined. They are not too far apart in resolution, but HD555 sounds quite a bit more textured and focused, whereas M50 sounds bland and washed out by comparison. HD555 has a much tighter bass without the M50 boom and bloat, much fuller richer mids (especially after the mod) and quite a large soundstage (again, after the mod - before soundstage was a bit closed in) with good depth, air separation and better imaging than what M50 can ever muster. I would say that if you want the warm, full, lush presentation and can't afford Sennheiser 600 series, than the 558/598 are excellent lower priced alternatives that are much better sounding than M50, or any other closed back over the ear headphone I've heard in their price range or lower for that matter (except perhaps for Orthodynamics). Still if you can afford it, I would suggest the HD650, or at least HD600 and if you are OK with doing a bit of equalizing, then the DT990 Pro is a steal in the sub $200 range they sell for as they will be a noticeable step up from HD558/598 in sound quality.
Edited by Pianist - 6/11/12 at 6:29pm