My current test bed has logged may broadcast hours using just two RF devices running at 600-700W carrier and about 165VDC of B+ before the modulator. The problem with adding more devices is you multiply the number and quantity of circuit strays, and this in turn winds up limiting your operating frequency. This is the primary reason those frankenstein rigs with dozens of fets barely work on 80 meters, and not at all on 40 meters.
If you want big power, you need big fets, which have higher input capacity. What I'm doing now was not practical prior to the introduction of SiC devices due mainly to limited Vds ratings and input capacity fo conventional fets. The thermal management problem is the last hurdle. The current PA runs about 93% efficient at 6.9 MHz. Add to that the losses in the modulator, around 4-5%, total DC to RF efficiency is on the order of 88%. At carrier that is approximately 30W per device, and it scales from there. At 1500W and average modulation, its around 77W per device and the thermal headroom starts to get thin. Maximum case temperature at 77W for the C2M0080120D is about 100 degrees C. The aluminum oxide insulators we use have a thermal resistance of about 0.315 degrees C per watt, so expect a 25 degree C rise right there. A reasonable heatsink temperature is about 50-60C, so we would need a heatsink capable of dissipating 260W of heat and exhibiting a rise of only 10 degrees C. This means we need a heatsink with a rating of 0.038 degrees C per watt, which is tough, but attainable under forced air with a bonded fin heatsink.
I'll try some of your exalted fet drivers this week and see how they do. If history is any indication, I expect them to slide off the board when trying to drive one of my fets.
+-RH