I heard it mentioned, outside of this forum, that the 6600 does not require external bandpass filtering for SO2R operation. If accurate, this would be another example of the “economies” and station simplicity created by this radio.
My club did a virtual meeting with flex. The flex representative explained that the 6600 has a 7th order bandpass filter on all contest bands. That's more than enough for SO2R at 100watts, if your antennas are 90° to each other and/or far enough apart.