[thesteve]

I don't think the voltage limit of the mosfets is the problem with supporting 6S in the ESC. Mosfets will keep operating just fine at higher voltages until they fail - and when they do fail you'll know cause you'll smell that lovely electronics smell. Without having one of the controllers to look at I believe the problem with higher voltages is simply a function of how fets work, in general you need/want a higher voltage on the gate(the on/off pin if you will). To get this higher voltage pretty much all ESCs have a voltage charge pump(booster) So even though you may have a 6 cell nimh pack connected the ESC is turning the fets on and off with 12 or more volts. I am guessing the charge pump in the Quarks isn't high enough to keep the gate voltage higher then the input voltage, and as a result the fets will not turn on properly.
The ESC above appears to have 36 fets (6 per phase) which is a very solid design. Even the Schulze U-Force which is a very good controller only has 18 fets in it(3 per phase)
Also the fets they are using are quite good - rated at 28 amps in real world conditions.