I meant maximum amplitude (Vpp) in the unfiltered output of the class E connected to a 50ohm load, not RMS voltage. You should still use a 100nF as DC blocking as well. The waveform shown in the manual in section 7.4, at the positive peak when the transistors are not conducting, should be maximised at the resonant frequency ?! Use BW limit in the scope if it looks too messy.
Maybe replace C30 with a trimmer, such as a JML06 ?
I'm rebuilding a 17m version to 20m soon, and will probably experiment a bit with this as well.