any fixes for impedance mismatch

If you use a single ended signal into a balanced input (XLR) you must ground the (-) pin of the XLR connector. If you don't hum will result.

The ten-to-one rule is just a rule of thumb to keep you out of trouble. Rules are made to be broken. If it sounds good, it is good.

Yes, circuit changes could be made to lower the preamp or raise the power amp, but then you wouldn't be using the same electronics.

Check out BVaudio.com, the Sound Refiner SR-10. Not sure this is the answer, but worth a look. I used one to great succes driving long cables from a passive pre....

I had a similar problem with Edge Signature 1 amplifers (single ended only) the solution for me was MIT Oracle interconnects they are modular so you can change the connectors however you please. And secondly they have a impedance switch that allows you to match the impedance between componates as needed. Allot cheaper than buying a new amp or preamp !

Good Luck
Definitive

Ground the 2nd XLR pin as Eldartford suggests and use the balanced input.
However, as your bass energy is there, you don't as if you're not suffering from energy losses, anyway.