From the base numbers embedded in the part numbers (7818, 7918) I would guess you are looking at the positive (7818) and negative (7918) voltage regulators for the chip stuff in the amp.
Find a handy dual opamp (4558, TL072, LF353, etc) and check to see if you get 18VDC at pins 4 (negative) and 8 (positive) referenced to ground.
Sometimes the positive regulator gets heavier use than the negative one, as it may be used to drive LED's and stuff so would run a bit hotter. I think that 133F (53C) is still well within operational limits.
Of course cooler is usually better for electronics. Manufacturers have a nasty habit of using the minimum heatsinking they can get away with. Make sure it is making good contact with the heatsink, and if it is insulated with a "sil-pad" rubbery insulator consider replacing that with mica & thermal compound.