Hmmm I don't know if this is the case but I'll just throw out a few possibilities out there if could or could not be any of those:
Mimby need to be left on 24/7 to sound correct, it has optimal operating temperature and people recommend at least 24 hours minimum before you listen to it.
I had Liquid Spark briefly before upgrading to Project Polaris. From memory I think manufacture recommend 100 hours burn in on Liquid Spark, also I do remember Liquid Spark being a little bit harsh / dry on my first impression. I didn't keep it long enough to hit 100 hour mark so I don't know.
I currently have Mimby running on USB and connecting to Asgard 3 and Project Polaris and I didn't find female vocal harsh, but treble do get loud on Polaris. And in general Piano sound is a little over emphasized.
Not too sure if the usb/optical/coax thing would fix it though. But you could probably give it a try.
Sorry I'm just throwing all the possibilities out there not sure if any of them would actually fix it LOL.