Dannyv:
I am not sure what impedance mismatch problem you have been hearing. I have been using a BAT-VK50SE to biamp Theta Dreadnaught for a few years with great results. My speaker was Martin Logan SL3 and recently, upgraded to Vandersteen Model 5. The input impedance of the Dreadnaught is 100k on XLR and I connect it to the BAT with 4M balanced interconnects.
I am not sure what impedance mismatch problem you have been hearing. I have been using a BAT-VK50SE to biamp Theta Dreadnaught for a few years with great results. My speaker was Martin Logan SL3 and recently, upgraded to Vandersteen Model 5. The input impedance of the Dreadnaught is 100k on XLR and I connect it to the BAT with 4M balanced interconnects.