Some of Denon and Marantz protection circuitry will lock up the machine requiring a technician to unlock it. No reset will do it.
Usually it is an Amp Stage issue (or possible other electrical) that causes this type of lock. (A High-Temp Fault will shut you down until it cools, but not lock you out.)
Unfortunately any further explanation would just be supposition based on guesswork.
I guess the big question I would ask is what your listening habits are? Do you push your system hard? Listen at high volume (SPL)?
To your comments about being disappointed in Sound (Quality?) I can only guess that you either don't actually like the sound of the DefTech gear, or you need to work on improving the placement and setup. That I'm not a fan of DT is immaterial: there are many fans that swear by that gear.
As a Marantz owner (same generation as yours) all I can say is that it is a good machine and does not have any particular SQ when used within it's ideal operating parameters.
The 4700 is definitely a lateral move, but that's not a big problem if you are mostly happy with the technology. It is newer by a couple years (2-3?) so you get a few new features not on the x012 series from Marantz.
From a setup perspective, when you connect your gear, make certain to not change the impedance to anything but 8 ohms in your AVR. Even though the 9080s dip down to below 5 ohms, they should not really present a difficult load as best I can tell...
Unless you were pushing them super hard.
Yes, you can set up your system to use the external amp to power the mains, center and surrounds. You will not be able to switch on the Denon 4700 Preamp Mode, though, if you are going to use any of the onboard Amp Channels to power rears or Atmos Speakers.
The online manual is easily accessible for both the 7012 and 4700 and can help you set things up properly.
I think the biggest question is: "What Happened?" Something tripped up the protection circuitry.
Did you have a storm? Power Outage or Lightning?
Did you short something in connecting your Speakers?
As I asked above... What are your listening habits?
Just to be clear, I just want to help you so you don't do it again.
Whatever shut you down could hopefully be repaired, or if you choose to replace... the risk is you do it again.
One last note as a Marantz owner. I know these machines can get warm. It is very beneficial to make absolute certain that you follow the installation instructions about distance above, around, behind the machine and not putting it in a closed cubby, especially with closed doors.
You can add some very inexpensive active cooling solutions very similar to PC case fans that will help pull cool air through the machine and keep it at a lower temp during use.
Heat is the number one killer of electronics. Where I said above, a temp-fault will not lock you out... but excessive and prolonged high temps can fry other circuitry.
I hope some of this might be of help.
