Vintage Luxman L-450 crackle and hum

Hi All - I'd be very grateful for your thoughts on a disturbing new development. This morning, after a nice and not very loud listening session, I stopped the last song and noticed that all was not quiet. The amp, a Luxman L-450 in pretty good shape, was making a kind of static-crackle hum, not very loud or low — like maybe middle C on a piano. I turned the amp off and back on: still there. I switched between all of the inputs (aux, phone, tape, etc.) and it was still there. Volume and tone controls had no effect. (It did not get louder when I turned up the volume.) What could it be? And what's the right way to repair it? I really like my Luxman and I want it back in shape!