At this point, after reading the forums and weighing different options, I am now leaning toward the Grado SR-225s. I have read that they don't need an amp the same way the Senns do. I ordered an I-River PCD and am waiting on an ebay auction before ordering the phones. If I can hold off on the amp for a month or so, I may do that.
Chris, I have owned Sennheiser cans in one way or another (as themselves, or as rebadged) since 1968. They have never managed to get their bass range anywhere near the quality of their mid and high. They do have bass, it's clean and clear, but it's definitely on the lean side.
Beyer is much better in this respect, and in case of model 990, I feel they could run for the best dynamic headphone title with it.
Grado takes a little from the old Koss school by having a deep, clean and convincing bass; it's not overblown as in many a Koss headphone of old (although I loved their Pro AAA I used to have), but also takes a bit from both Sennheiser and Beyer in that they offer very good clarity across the range. Where they truly shine is in the way they integrate the entire spectrum into one whole, and they do so seamlessly. The only quibble you might have with them is their, shall we say odd design, quasi-retro but not retro.
SR-60 is a masterpiece - for the money, I can't think of anything that will even touch them. SR-225 is the same as SR-60, only more of it. Integration is about as absolute as it can be in real life, nothing pushes, nothing pulls, nothing stands out, yet they manage to bring out the spirit and emotion of music - not at all easy to do. Sure, the price is higher, but I feel it's well worth the asking price, and in fact, I am about to augment my SR-60 cans with an SR 225 pair.
Cheers,
DVV