Fan on Amp Unbeatable

Another idea to consider: take two of the largest diameter DC fans you can find, the larger the better. Wire them in series to a large enough DC wall-wart of the same voltage as just one of the fans and go for 50~100% over the current draw of just one.  Pad the bottom (intake) sides of the fans with soft weatherstripping foam to isolate vibration from the fans, and lay them over the top of your amplifier above the heat sinks. 

I did the above with an old receiver many years ago, and it kept the amplifier section from overheating, even when driving hard loads at high volume levels. 

I did something like sleepwalker described with my home theater processor and my Peachtree Nova 300, except I used a usb fan. You should be able to connect the usb fan to a phone charger with a usb outlet. 

Anyone wanna buy a noisy Marantz 500?

If you are serious about selling your Marantz 500, they are currently asking big bucks for them right now. Go look on the bay. Many of them go overseas ( I do not know why ), but they do.

@pawlowski6132 - I can't stand any audio equipment with a fan. The noise reduces the S/N ratio tremendously.
If your amp really is worth big bucks (as @mrdecibel says), my suggestion is, sell it and start fresh. There are a lot of great modern SS amps around, and very few have fans.