"Can I reduce the output power in half by eliminating two of the four output MOSFETs in order to make the power amp like that of the 100W 3510 model" Simple answer: NO. The mosfets are handling the power of the amplifier. ie: The voltage rails. Not too sure how to go about it but the key is how hard the preamp is driving the power amp.
You do not 'have' to have a scope. You can see the waveform with your meter set to read Volts Ac. In your case the tremelo is stopping. So if you monitor the transistors you can see it. I would start with Q10. Base & or the emitter.