You've been thinking about it for years. Sell the Senn to Andrew and take the plunge!
(this message brought to you by your local audio pusher)
Aside from the quality of the pads (which are very good and very comfortable), I'm not sure I'd rank them sonically above any number of similar mods I've heard. Initial impressions: good subbass extension but midbass hump, flat-ish mids but goes into a few treble peaks that get worse at higher volumes. The upper part reminds me a bit of a grado actually. Overall sound is slightly V-shaped, which would probably appeal to the largest base of consumers.
Still agonizing. I'm concerned (convinced, actually) that I'll just want to buy a set of HD600s again later. Arrgh. I'll have to listen to them some more tonight, and maybe A/B them with some other dynamics I have kicking around.
On the "pro" side is the fact that the SRM-T1 is considered the bare minimum for powering an O2, so it would be a step in the right direction, if an O2 is in my future.
I'm trying to decide whether I should drop the cash on new PA gear tomorrow. Long & McQuade is having a sale on some used rental gear and I'm very tempted to pick some of it up... but that's going to be at least a grand in gear. Ugh.