Power Amplifier Longevity

Hello,
I can tell you from experience that having a really nice multichannel amp for home theater is awesome, but also very expensive. It sounds like you are trying to get two channel music an surround out of the same system. Marantz is my favorite Pre/pro. You have to realize that it is a processor. Even in pure mode it wants to color the sound in two channel. For now if you want higher end I would look at NAD class D for the center and surrounds. The processor can manipulate the sound to make the speakers blend together. Personally I would go with an Emotiva 5/4. Call them or look at the website and you will get it. You can have 5 high powered channels for the center and surrounds and use two of the slots for the 4 heights/ atmos. No risk because you can return it if you hate it. 

Now for the two channel. Now or later you would get a two channel preamp with home theater bypass. On the lower end you can go with an Emotiva USP - 1 ver 2 for $1100 retail and $750 eBay. Or on the high end Hegel for $4000. I have the Emotiva. I like that it lets you add the subs to your two channel setup and uses balanced input/ outputs if you want. 
Here is how it works: 
To listen to your system in stereo for music or TV you turn on your two channel preamp and stereo amp. High power great sound and pure as silk. When you want surround sound you turn on you Marantz Pre/pro and your multichannel amp which can be turned on using your 12v triggers along with your two channel setup if you choose. Push the HT bypass button on the two channel preamp and the Marantz controls the volume and sound of your two channel system. You can even program some of the two channel buttons on your Marantz remote so you do not need the 2 channel remote all the time. I love separates. It gives you control of your system. Later you can add a DAC to your two channel system for more better digital sound. That is a whole different thread. If your not into the music or two channel stick to my first response. Also everyone in this chat is correct about the caps on the older amps. Your speakers are very nice and I would be careful about older stuff. If you do by used please have it checked out by a professional before you hook it up. If the caps are bad or the gain or bias is set wrong you can do a lot of harm to your system. $50 can save you thousands. 

One last thing Brian339. You mentioned you might get height speaker for Atmos later. If you do not want to add the expense for the Atmos amps right now you can upgrade and add them to the Emotiva box later, You do have to send it back for them to add the amps. The amp box has seven amp bays. You can buy a five channel amp in a seven bay box for expansion later. The last two bays would have lower powered stereo amps for the heights/Atmos. Due to shipping cost just have them installed now so you are ready. Worst comes to worst you can power a second room or some out door speakers for now until you decide what to do about Atmos. The Marantz receiver can do all of this without breaking a sweat. 

  iv'e bought a used 1990's arcam delta power amp, sounds great and looks mint  .if i   h e a r   a bad sound than i might worry about caps. it has no moving parts and if it's a quality piece usually it's been well taken care of,if not than after 30 years you will see signs of abuse or humidity.if it looks good and mainly sounds good don't worry about it. lexicon's are a great  choice and sounds way nicer than most of today's amps and will pair well with modern dac's preamps and speakers.go for it.

I'm with Erik. No AV piece made more than a couple years ago is a good bet for longevity (excepting Theta Digital).  The Anthem would be a nice upgrade in sound quality over the Marantz, as well.`One of the most disastrous adventures upon which I ever embarked, as far as my marriage is concerned, was me trying to repair my aging Conrad Johnson PV 5. I didn't think to fully test the transformer until after I'd replaced all the caps and several other components. I personally, will never buy used again. The risk is much higher than people think and the opportunity cost is huge as well.