#1
i want to buy a vox valvetronix 100 watt amp head to plug into my marshall cabinets.

my question is would they be compatible with each other.
my speakers can handle 120 watts
#3
yea. Any head will work with any cab, as long you have a 4 Ohm/8 Ohm/16 Ohm jack on both. Idk what you would have, since idk the config. of your cab.