Not sure what amps you have, but this one will cover most amps (hand full of bass amps, 5Amps wouldn't be enough). The votlage display is pretty accurate as well. That is the biggest concern with a variac you can overvoltage your gear if you are not careful.
https://www.circuitspecialists.com/...MI3e7i64eh-gIVUhXUAR0Vag88EAQYAiABEgJ5IPD_BwE
The one you linked to is 20A and overkill. The voltage readout is not going to be super accurate either, but should work. If you are lucky you can score a vintage USA variac for like $20, but then you need a killawatt or something like that to know the voltage.