If you look at what is inside each component you will see a few things that stand out. Circuit boards, power supply capacitors, transformer, etc. Power supply caps are not that expensive like Nichicon, resistors and capacitors vary in cost depending on the brand and what you are trying to accomplish in sound. The circuit boards can go bad over time depending on the heat. All that being said, almost all components can be repaired/modified/upgraded. So IMO servicing is available and it can be reasonable.
I would not trade in the Marantz if you like the sound. I would only trade it in if you find something else that you like significantly better. If you think that it is not up to today's standards, send it to me for upgrades.
Trading it in for a newer amp does not mean that you will get better sound, just maybe different and that will not mean longer life either. Most components are similar in build so there are a few common upgrades that can make a significant difference for very little money. A recent example was a DeHavilland preamp I just worked on. I had already changed the transformer, volume control, added V-Capacitors, etc. The owner loved the sound but it was slightly warmish sounding and lacked a little dynamics. Changing out two resistors from the Allen Bradley to Audio Note gave it the snap he was looking for, quieter, more dynamics, more details and ore open sounding. You won't get that from any cable change that is for sure. Less that $50 with parts.
Most of the comments here are from people who don’t really know that upgrades can be more effective and not that expensive and the component can be taken to another level of much higher priced components. Most components are built to a specific price point and not an all out build with the best parts, etc.
Happy Listening
