I got mine at the end of January and it started having the "cracking on left channel" issue about two weeks ago. I've already returned it sadly, so I can't look up its serial number anymore.
One thing I was wondering about: I had the amp plugged into an automatic power strip that kills the power when the main device powers off. I suspect the amp did not particularly like that, as it produced popping sounds whenever it lost its power without being turned off separately first.
I will admit I have no clue about how these amps are designed, and whether it's even possible that that has anything to do with the left-channel issue, but I thought I'd mention it in case it turns out to be a factor distinguishing the defective units from the surviving ones.
Mine had the left channel start making noises about a week after I bought it. It happened while it was left on overnight, not power cycled. The unit had only been turned on and off a dozen times or so, always with the switch. That being said, I fully expect electronic devices to be able to be shut down by disconnecting the power without failing. So no, your use of the power strip likely had no impact on your amp.
I am pretty amazed by the apparent defect rate.
~100 PA5 fails reported here. If Topping sold 100,000 units and assuming these are all valid defects, this is 1000 Defects Per Million which is quite horrible. I don't think Topping sold nearly 100k units, and I think many more defects are unreported, so probably much higher than 1000 DPM. I also believe all of the reports here to be Topping defects, not user error or some other issue. And the issues are not nuisance issues. This product had real issues from the beginning.
Also, multiple people on this thread had two PA5 fail. Some of these second fails are replacements that failed. This makes it even worse since it rules out the possibility that ~100 units from a bad manufacturing slipped out o the factory and made it to ASR members. One of the worst-case scenarios for a Quality/Reliability team is to ship a replacement that fails in the same way.
This appears to be endemic to the product.