I've used SR80s for about a year and currently have recently bought 4 sets of headphones to find an upgrade, including HD600s with a cardas cable, which are very deceptive.
I've heard the HD600s before and liked them so much i knew i'd get a pair at some point. To me they have a very mellow, smooth sound which works very well with classical, but they don't work as well with anything that needs the bass to kick.
I'm not sure exactly what 'Ambient/down tempo electronica' is so i'm going to assume Orbital/KLF chillout style music which i think sound good on the HD600 but don't envelope me as much as the other headphones i'm listening to, the SR80s are definately more aggressive and in my face and are a sharper sound than the HD600s, but yet not as detailed.
In summary, if i want to sit quietly a few rows back and listen to music the HD600s are a great choice for me, but if i want something to grab me by the hand and yank me onto the dance floor they are disappointing.
The deceptive part is that while i say they are disappointing, if i put on another set of heaphones straight after the HD600s (SR80s included) the headphones sound closed in and harsh and even more disappointing than the HD600s. It takes a couple of songs for my brain to adjust to the new sound signature and for the strengths to kick in.
But enough about me, why don't you feel you can use the SR80is for Ambient/down tempo electronica music? And what source will you be using, the HD600s need more power to run properly than the SR80is so if you're using an MP3 player and need a good volume they may not be the right choice anyway.
And neither the PX100s (which i also own) or the 201s(from what i remember of the ones i bought a while ago and sent back on the same day) will give you a useful indication of what the HD600s are like.