Chanher,
If you don't get the used SD MixPre 6 in the YS, I recommend the Zoom F6. I love mine. Compact, solidly built, less expensive than Sound Devices recorders, preamps are super clean, and you get 6 XLR mic/line inputs which can be recorded as ISO tracks, or combined into a polyWAV file. It also does 32-bit float with multiple ADCs like the new SD recorders. The only possible downside for your use might be the lack of 1/4" jacks for soundboard patches, but adapter cables are easy to find or make.
Repeating what others have said: The quality in the new Sound Devices MixPre and Zoom F series recorders are not something you would ever need to mod. I used to own a Tascam DR-70D that I had modded for better performance, but my F6 blows it away.