SSB modulation was fine before I started having the feedback or whatever issue it should be called when testing the Carl Built amp (not its fault!) with the Q5N2. Ever since then, the SSB modulation can best be described as crunchy, choppy like when you're using a crappy amp on SSB except this is just straight radio. The modulation on AM and FM is still crystal clear. I have tried various power levels, modulation levels and two different mics, multiple brands and lengths of RG8x patch cables, and two different PSUs (well, three if you count running it from a battery). The Q5N2 is connected to a dummy load and I am listening to the voice on my President Lincoln II+ V4 that is not connected to an antenna. The only thing the Q5N2 is connected to is the dummy load. No antenna switches, SWR/power meters, etc. SSB modulation on the Lincoln is clear both without and with running the amp.
I replaced the four IRF520s with Vishay brand ones from DigiKey and just did the bias voltage adjustment.