Oh you can perform many changes but that thing will sound different from what it does now. Not worse but different. 40 years is a long time and parts do age and by this time you got accustomed by its slowly degrading sound, normal. For start replace all electrolytics in amp, and speakers, that only will revive them. Same values, voltages, capacitance, temperature (especially temperature) for keeping at least the character of the units. If you want to go deeper and to unknown, some diodes, resistors, regulators. These changes are not expensive but require a bit of studying before any attempt. I recapped my 1978 tuner, did not alter anything, it only got better. Try to get amp's service manual, it would be the most helpful. Whatever you do good luck and have fun.
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