I have a denon pma 500v amp, has distortion in right channel, both speaker output and headphones, what should i check for first?

You just need to replace your volume knob for a better one , a couple of bucks 10 dollar

Look around for a subwoofer which has speaker level inputs.
Something like this;
https://www.parts-express.com/dayton-audio-sub-1000-10-100-watt-powered-subwoofer--300-628

The only safe way to do this us to connect the sub to one of the other speaker outputs using a "speaker selector" switch. For example, you would go out of the receivers left speaker output, into the selector switch then out of the selector switch into the sub and the left speaker. There really is no other way.

Hello there, I have the same amplifier and now it has the same problem on the right output. Have you found out how to fix it? Thanks!

Anything that distorts both channels has to be something where both channels go in and out of it. Is the main amp an IC Power Amp such as the one shown below. If it is replace it.

hi.
Did you checked balanced impedance?load speaker impedance vs PA output?or PA might over driven,causing a trouble on the pre-amp to power amp circuits and components damage,
need to recheck related components on the pre-driver up to main output section,

Note: That,blowing of fuses with the same rating is an indication of trouble/shorted in a circuits that driven on it,

Thanks and good luck

microhit

The good news is that it's in one channel, which means you have the right channel parts to compare it with. Now the bad news whatever causing it and it's likely to be a semi-conductor (transistor or IC) has to be replaced. However finding it is easier than you might think. For a start it's going to be hot to touch. And you can get get it to stop being hot, while the amps on, so correcting the fault, for a while. You do this by spraying it with Servisol Super Freezer spray. You have to have the amp running for this. Find the left channel amp and wait while it goes bad. Then spray (one at a time) any part you suspect. If the sound returns to normal, you have found the faulty part! To prove it spray the same in the right channel. That shouldn't change.
You can get the freezer spray from your local electric parts shop (such as Maplin UK).

That model shouldnt be stuck in stand-by
Are all the speakers connected?
If it wont properly turn on then there is a fault being detected. Could be disconnected speakers / bad speaker or a power supply issue

Try a set of headphones and see if the high end works in those.
Also , is the treble on the amp turned way up?
Get back to me
Thanks

If you understand electronics then work backwards from the speaker outputs to the imputs. You need to make the channel buzz by touching the circuit, you can use a small screwdriver. Best of all is a signal injector, you can buy one at a electrical service shop (Maplin UK) they can be cheap for a very simple one.
Compare the results with the good channel. When the sound gets weak you have located the area in which the fault lies.
If not then off to the repair shop.