First off the output of the mixer is anywhere near -6dbm its going to be distorted.
I'd expect -10dbm max.
The mpsh10s I have not tried as they have not arrived.? I'd expect to have to pair them up
as their power handling is about par with 2n3904s.
Also my test includes the q90, predrier and driver that should be over 44DB of gain
or you will never make enough power to make the IRF510s run.
The 3866s had the existing bias.? Also I have all of the peaking suggestion in place.
Allison