In your case as described, you shouldn't have a problem with either the 60's or 61's.
4060 > UGLY BATT BOX > M10 should not overload in the situations you describe. I've only overloaded the input of the original R-09 with the 4060 output a few times- one was a way, way too loud festival arena show (even with earplugs), another was a club with massive subs under the entire front of the stage and I had the mics setup at stagelip directly over and very close to them. But the input stage of the M10 can take a hotter signal without overload as I understand it. So you needn't worry about overload with the more sensitive mic, and that extra sensitivity might allow you to just use the UGLY BATT BOX instead of the 9100 for more things than you would otherwise.
More often than overload, I've encountered the noise floor of the 4060, but mostly on nature recordings where a lot of gain is required. Even on quiet material like classical string quartets and super low level 'threshold of hearing' segments in some modern percussion pieces, the HVAC noise of the room has been higher than the self noise of the 4060s. I have yet to use the 4061s I picked up, so I can't say how much higher it's noise floor is, or how much of a problem it might be for typical music recording use. Some have reported it as overly noisy for music with very quiet passages like classical or maybe acoustic folk, others say no problem.
Short and long is I still think 4060 would be preferable for you, but not so much as to keep you from jumping on a good deal on 4061s if you come across one. If buying new the price should be identical, so go 4060. This was my reasoning when I picked up a handful of used 4061s on the cheap a couple months back.
I've had great luck with quiet acoustic and loud amplified music with 4061's. Especially since I went 24 bit and can amplify without hiss.
I don't doubt it at all, but 24bit doesn't do anything to help the 4061's self noise. It gives the recorder a wider dynamic range which allows you to cleanly boost the signal it records, including any noise that is already there from the from the room, mics, or preamp, whichever is loudest. So recording more bits means a greater potential of revealing the self noise of the mic, which might have been hidden in a 16bit recording, depening on recording levels. In that way, your not having noticed a problem with self noise of the 4061 is a good endorsement of it's higher noise not being too much of a problem.
One word of caution though - these mics have generally already been re-terminated from the original connector to a mini-Lemo thing and have often had the cable shortened at the same time. Not a problem for low-pro outings but you might struggle if you want to get them up high on a stand.
I regularly extend the ones I have terminated in miniplugs with a 6' stereo mini cable to either the UGLY pres or the Niant PFAs, which works fine without introducing noise as long as the connection is good. I usually gaff tape that. I'm getting ready to convert most of my miniplug/jack gear to mini-XLR for more secure locking connections.