Hi there, another classical person here (pianist and choir director / singer). Just chiming in to agree that there's no need at all to upgrade from your MixPre; its preamps are already outstanding. The same could be said for the Zoom F series recorders, which I am a huge fan of.
Like others have said, the biggest improvements will be made by better technique, and then second, better microphones. Speaking of which: how are you recording yourself, and what mics are you currently using?
The people on GS Remote who say that you should be using Sonosax, DAD, Pueblo or other preamps are generally people who are professional recording engineers and who own the finest microphones available. At that elite level of technique, experience, and equipment, you are refining to the nth degree, and top end outboard preamplification and conversion might get you to the mountaintop.
I am very fortunate to have recently acquired some DPA 4000 series mics which are definitely in the world-class category, yet I have zero desire to replace or supplement my Zoom F6. My technique is the area that needs the most work, so that's what I'm focusing on.