I repair Onkyo's as a hobby (over 100 units repaired). As noted in this thread they have had a couple of issues in recent memory - HDMI DSP chip failure and HDMI capacitor failure. In their defense, the HDMI DSP chip failure wasn't their fault, it was their supplier (TI) and Onkyo/Integra did step up to extend the warranty for affected customers. The HDMI capacitor failure was caused by heat degrading the capacitors that supply power to the HDMI board. This one can be caused by poor ventilation but ultimately, Onkyo bears some responsibility since they are the people who laid out the board and often didn't install a fan (most designs have a provision for a fan but Onkyo didn't install it, likely to save cost).
In your case, I would strongly suspect that you have some failing capacitors in the amplification circuit.
The big issue with Onkyo's WRAT technology is that it generates a pile of heat. Even with good air circulation, their designs can get very hot. Unfortunately, heat erodes electrolytic capacitors and there are a couple of places in the Onkyo WRAT designs where this causes issues on the Amp side. On both of your units the issue is likely the failure of one of these capacitors on the pre-amp board. This board sits very close to the main transistors so it is subjected to a lot of heat. What Onkyo should have done is to have used capacitors rated for higher heat in this area but again, this would increase the cost. The capacitors Onkyo uses are rated at 85 degrees Celsius. When I repair them, I upgrade these caps to versions rated for 105 degrees.
On both units the capacitors causing the issue are likely C5010 (FL), C5011 (FR), C5012 (C), C5013 (SL), C5014 (SR), C5015 (SBL) and/or C5016 (SBR). Each of these is a 47uF/50v "Audio" rated capacitor.
There are actually 3 47uF/50V capacitors on each pre-amp channel and all of them can fail due to heat so I typically replace them all. The one I think is causing the "crackling" is used as a filter for the incoming signal but the other 2 sit right in the middle of the pre-amp circuit. When either of these two blow they usually cause the main transistors to fry as well (along with a couple of other transistors and a couple of resistors).
I know this isn't good news and while Onkyo bears some responsibility for these failures (by using lower heat ratings and removing the fans from the design), the root cause is usually that the owner has placed the receiver in a location with poor air circulation. This can be a problem for any amplifier regardless of brand (I have fixed NAD, Pioneer, Yamaha and Harmon Kardon amps that also got too hot)
If it makes you feel any better, what got me into this hobby was the failure of my SR605. It worked perfectly for 4 years in a well ventilated environment but then we moved. In our new home I didn't have space to allow air flow around it and shoved it into the bottom of the TV stand with about 3/8" of air space on top. Making matters worse, my new DirectTV remote no longer offered the ability to power off the amp so I simply left it on 100% of the time. The poor amp ran like this for 5 years before it finally blew. I found 3 failed amp circuits, completely degraded HDMI board capacitors as well as other components that supplied power to the main cpu and DAC circuits. This sucker took me almost 6 months to completely diagnose and fix all the issues. Point is that it can happen to anyone.
Any replacement amp from any manufacturer will have issues if you don't supply at least 8 inches of free air space above the Receiver and on the sides.