The 600 is nice and all, but it just lacks the clarity and detail that I get with the 701. Where it comes handy is the strictly dance oriented electronic music which just doesn't work with the 701. The 600 is veiled, compared to the 701 atleast, and there is allot of electronic music that does work with the 701, downtempo, dub, some idm, ambient, modern classical... Allot of the time I will be listening to something with the 600 and wonder how it will sound with the 701 and end up keeping the 701 on my head. I did just buy some 2003 Beyer DT 880's [this is what got me to think of selling] to see how those will work, had the 2005 600ohm model, didn't like it. In my mind I will either sell the sennheiser or the Beyer... Do enjoy the warm presentation of the Senn's though.
My wallet is hiding somewhere, so no more spending money for now... Have a bit on reserve to get a decent DAC anyway.
The amp I'm using is the X-can v8, which I think works great (though haven't compared it much anything else at its level), its a hybrid ss with tube output stage, have some NOS sylvania tubes in there which are pretty clear/detail oriented. I've had some tubes that have some nice lows, but they messed with the midrange/high's, making them harsher/less pleasant.
As for the 701, that phone is something else. It's less clear, but it's just so good, its presents the sound in a much more controlled manner than the 501, sounding a tad bit veiled but really grabbing ahold of the music. The 501 sounds "raw" compared to it (don't mean that with a positive or negative connotation).