1) if you have no problem with power amp eventually clipping but "want it to clip like a Class AB amplifier and not a Class D one" just add a couple (or a string) of plain diodes across its input, so "they" clip first.
Diode clipping has quite a rounded and smooth edge .
2) if you do not want the power amplifier to ever clip, no matter what, add a non clipping limiter between preamp and power amp.
Scope output so you have as much as possible without squaring peaks.
I suggest the simple and elegant Crate/Ampeg opto limiter, which stops waveform peaks a couple Volts before reaching positive or negative peaks.
3) set preamp to maximum clipped output, balls to the wall, add a screwdriver adjustable "master" at power amp input, scope output and sllloooowwwwllllyyyy rise Master, starting from 0, until you see the wonky tube preamp waveform starts to squarewave on peaks (indicating now the Class D amp has started clipping by itself), then back down a little.
Now nothing the Preamp does will clip the Power amp.
FWIW that is the method I use.