I do have three shows on LMA (GPN, Brett Dennen, TLG) with CCM4 > stock 661 and several other shows on Dime (Cranberries, Rodrigo y Gabriela) with the same setup. One thing to keep in mind is that the 660 required modding to make it suitable/usable for loud concert taping, but the 661 does not. So if you go for the mods by Oade, you should do it because you want the sonic difference of the mods and don't mind paying $100-150 more for it (I bought my new 661 for ~$500 on ebay back when they had bing cashback). Some may argue that the noise floor is lowered by the mod, but no one has objectively measured that so make of that what you will.
What I can tell you from my experience is that the noise floor on the stock 661 is very low. I've taped shows with very conservatively set levels (peaks between -20dB and -12db) and after normalizing to 0dB the noise floor is still very low. Unscientifically, I think the 661 has a noise floor comparable to the 7xx series (which is very low), because when recording in 24bit I have a ton of 'room' to raise levels without introducing noticeable hiss. But listening to tapes out there with modded and stock 661s, although not scientific or definitive, is still the best way to get a feel for how you like the sonics of each.
Either way, I think you'll enjoy the 661, good luck!