As someone who owns and enjoys both HD600s (with cardas cable) and SR225s, I feel the need to present another point of view here, at least about the sound characteristics of Grado cans. Build quality is another story, but one that's less important to me than sound quality. While I understand why some people feel that Grados are too bright and lack depth--in fact, I felt that way myself for quite some time--I've come to hear things very differently in recent months.
First, the Grado sound changed dramatically for me when I reversed the bowls. This took away some of the extra shrillness and added considerable bass. Second, once I got Millet hybrid amp, the Grado truly shone. Not only is their treble detail and ability to portray texture obviously superior to that of the Senns (which was always the case), but they have a kind of glorious, very liquidy midrange that really defines "audio heaven" for me with many recordings (Sarah MacLachlan's "Fumbling Toward Ecstasy" is a particularly good example of this). The Senns sound much "drier," and I use them mainly to tame extra thin, trebly recordings (REM's "Automatic for the People" is a good example of a great HD600 listen).
Personally, I think the 225s (and presumably the HF version as well) are a HUGE audio value--for me they sound better than the HD600s on most recordings (especially the richer-sounding ones), but cost a fraction of the Senns's price (particularly when the HD600s were still current). While I respect and value the opinions of those who disagree, I do wish people would be a bit more relativistic when they make pronouncements about value.
Actually, I'm seriously considering taking the big jump to the RS1s, in spite of the frightening price tag. If they are even a little bit richer sounding (to me) than the already-lush 225s, it would probably be worth it!