Previous I had no separate AMP. I used only Denon AVR 4400h. It used to heat up and stop when I play music at 85dB to 90dB for 30 minutes or so. That's main reason I bought Parasound A21+. With Parasound A21+, it doesn't stop but recently I'm facing issue with it it's heats up so much and struggle to pump, the music stops 1 second or so in-between when it struggle push when I play music at 85dB for 30 or so. Why does this happen? I'm so disappointed now.
Those speakers dip as low as 3.2Ω and that could be a heavy load, but that dip is very transitory in nature. I'm trying to figure out why you're having trouble, so perhaps you can describe the situation better.
When you are measuring that 85dB to 90dB level, is that at the listening position or is it nearer the speakers? How far from the speakers do you sit?
In my room I've got Reference 1's (which are probably a tiny bit harder to drive) sitting exactly 8 feet apart, and I sit in front measuring 8 feet from my head to each speaker. I have used both an app on my phone and my son's Radio Shack decibel meter, and I get 70-80dB
at the listening position very consistently. My amp is a Luxman L-505uX Mark 2 class A-B integrated amp, rated at 100w/ch @ 8Ω. It never misses a beat, never approaches heat protection shutdown or clipping.
It would be highly unlikely, but are your speaker cables capable of handling the high current that your magnificent Parasound A21+ power amp generates? Are you in a large room sitting very far away from the speakers? I'm not accusing you of doing anything wrong, just trying to help you diagnose what could be causing the problem.