Definitely not STA-6. You have to consider the max power, not just power to each speaker.
Even though our design goal is for ST-10M to be better sounding (besides having more power) than ST-10, we do have a few customers still prefer ST-10.
If you speaker is 2-way, I am not sure bi-wire has the advantage. If it is 3-way, may be it make sense to do bi-wire.
My personal preference is to go with ST-10M.