I record mostly opera recitals (singer(s) + piano is the most common) in nice halls and sometimes conductors' living rooms, etc.
I have done a lot of shootouts where I have identical matched stereo pairs of mics set up in front of a soprano and we swap out one piece of equipment like a preamp, then see if she and I can tell the difference.
I personally would stick to your MixPre-6-II.
As you know, the order of importance in terms of what makes a good recording is something like:
-having a recorder that doesn't break down/have a bug
-singer/performance
-hall accoustic
-mic placement (how many and where)
Then lower on the list is:
-mics
-mixing skills
Then even lower is:
-preamps
-converters
What does your mic locker look like? How are you setting up?
My experience with shootouts is that yes, some sopranos have golden ears and can hear when we swap one good preamp for another (and tell me that, for example, tracks 1 3 and 7 are preamp one but 2 4 5 6 are preamp 2). Same with converters.
However, these are *tiny* differences. Even the difference between two flat small diaphragm condensers like Schoeps MK21 vs Line Audio CM4 are very small to their golden ears (though they can still reliably tell them apart) and often invisible to my ears.
Your MixPre-6-II, unlike the sonosax, is 32 bit meaning you don't have to take an extra minute to set and adjust levels while you're also trying to perform. My guess is that you could swap it for the 'sax, but you will get much farther just buying more Schoeps or whatever you like, placing them better, and getting into a better hall, or doing whatever else you need to do in order to feel great on performance day.
FWIW, the sopranos I work with very slightly prefer the transformer preamps of a MixPre-D into the MixPre-6 via Aux ch. 5/6, instead of the MixPre-6's own preamps. This is based on my tests with Schoeps CMC1-MK21 or Line Audio mics (CM4, Omni1) in the usual opera spot position, about 4' in front of the singer.