From what you describe to me here, it sounds very much like the power switch has failing contacts. I have replaced several of these in the past as a Marantz Service agent. If there were to be intermittent connectivity through the power switch, you may well get pops from the amp as it connects/disconnects or arcs across the contacts.
If you are happy with leaving the amp on all the time(as you have been with the switch taped in), then a local tech to you could easily jump the contacts of the switch for you. I never turn my amp off... been like that for nearly 10 years. I think they actually last longer if you dont put them through through heating cooling cycles from power on/off. Replacing it is not a difficult job, you may have to wait for a short time for him get the switch in for you. If I recall correctly, they are fairly common type so he may be able to get one easily. if not, just jumper it. It may be an old amp... but is considered collectible by many these days. They don't make em the way they used ta :)
Happy to answer any questions that you may have about this. Please hit the ACCEPT button if I have answered your question. Cheers