I'll try to help, we may be looking for the same thing. I think the RS6 is very warm sounding but also slightly dark. In that regard the M60 is quite bright and again, the 603 is right in thr middle. These diferences are not huge, but you can hear them easily. In the "punch" department the M60 has the greatest bass extension, the RS6 would be 2nd and the 603 3rd. Articulation...man, all three are great but I think the M60 has a slight edge in that too, It seem it doesn't need to work hard at all. The RS6 and the 603 need to be driven harder to achive that prescence. Neautral presentation...boy, that's a hard one but I'll give that to the M60, although it has a forward sounstage presentation, the RS6 is more laid back, same as the 603. I think the RS6 is the more open one, the 603 is also open and the M60 is in your face. I will also say the M60 is the least boxy sounding. This is aparent only when played loud...very loud. Also, the RS6 is very linear is the low frequency range, I could not detect any bass note preference which is exactly what I read, and that is important to me. Then again, The M60 and the 603 are very good at this too being the M60 the hardest to control in a small room and that is why I may not get them because I have a 11x13 room, too small for such a powerfull speaker. Built quality can go eithr way, all very nice although I would say the RS6 is absolutely beautifull (dark brown or black).
I've tried to answer every question you asked and I sincelery hope this helps. One funny thing, the salesman, a very knowledgeable one, had me listen to the Monitor Audio GS10 first and I could not believe my ears. So far that is the best speaker I've listened too, but at nearly $2000 is out of my budget. If you have a big room, maybe more than 15x15 I think the M60 would be great. It'll fill the room nicely. I think you can't go wrong with any of these spaeakers, but just to give you my opinion the M60 is the one I like the most but it may be overkill for my room and between the RS6 and the 603 I would choose the RS6.
osvi
p.d. The cd's used for all tests: Dave Mathews (Crash into me, Song #40), Jonh Mayer, Joshua Redman, Tribal Tech, Sting.