You sure can, but it will sound much better with the line voltage cut-back to around 115VAC to 117VAC.
Get yourself a scalable bucking transformer. You can use a variac, but those are heavy, bulky devices. Plus, you can inadvertently crank the voltage too high; ruining the power transformer. I don't run any amp over 117VAC, but doesn't mean 120VAC will damage them. Vintage amplifers should never see more than 115VAC. Many homes see a wall voltage in excess of 126VAC. Ours gets as high as 129VAC! i.e. "Nashville" voltage.