It’s hard to find data on the resistivity of wood in any easily interpreted form, and MDF even more so. I’ve seen figures of 10,000 Meg ohms/cm for pure wood at 12% moisture content, dropping to 10 Meg ohms/cm at 20% moisture content. The glue layers in plywoid are supposed to be less resistive than wood. MDF is held together by Urea Formaldehyde, but again, initial searches give little info. Plus I have no idea how thickness affects the readings because you effectively get a lot of resistances in parallel, which gives a lower overall resistance. Moisture absorption from the amp sitting around unused in a damp or humid environment would be my biggest concern. It doesn’t take more than a few mV DC in the wrong place to start causing noisy pots on the amp, and getting towards a volt can affect preamp tube bias etc.

Even old fibreboard turret boards can be affected by moisture or other conductive fluids that have been spilt on them (beer probably being a culprit here), that may appear to have dried out but leave behind a patch of residue that’s far more conductive and causes operational issues with the amp.

There is also a much greater fire risk with MDF than say glass fibre.

I’d suggest that whilst it may be a cheap material to use if purely prototyping on a test bench, any incorporation into a proper amp is best done using standard materials.