Concerningly loud pops in vinyl playback

I have a problem I've been unable to diagnose yet, so I thought I'd gather some ideas. This is for a 2nd vinyl system in my girlfriend's home, not my main system. I've never had any issue like this in the main rig. 

We're all used to annoyingly loud isolated "pops" from surface defects or perhaps static discharge during vinyl playback, but usually these are only truly annoying during quiet passages and tend to become relatively "outgunned" during heavy loud music. However, very occasionally (once every several records) I will get a POP so loud it is concerningly startling, even during loud playback of rock music. I don't like the tweeters being subjected to that, though they're fine so far. And it hasn't tripped the VAC 200iQ's protection circuitry yet, so there's that. Still, I don't like this. It's happened on the left and on the right speakers, so it's not isolated to one channel. Once it occurs, it is more likely to occur again within several seconds (and again), until I take the record off and start over. It seems to occur only starting at a certain high volume level on my ARC Reference 6 preamp (40 and above). The records and stylus are clean, and it's now occurred on multiple different records that I KNOW aren't the problem. So my suspicions are down to: 

• Static discharge (I have a  record brush with conductive fibers and use anti-static sleeves)
• Really bad overload reaction in phono and/or preamp, possibly prompted by static discharges and possibly because the gain is set too high in phono stages (I have the Hagerman set to 60dB for 0.8mV which I thought would be OK)
• Power line noise issue of some kind / EMI / RFI
• Tubes in the ARC Ref 6 going bad. Maybe PSU tubes? But it really seems to be instigated by a volume level on the Ref 6 going above 40.

This rig is kind of like my playground, so I have lots of gear at my disposal, much from the main rig. I've already swapped out many different components and had the problem reoccur on each of:

• Different Speakers (Tannoy TD10, Tannoy Glenair 10)
• Speaker cables (AQ Rocket 88, AQ KE-4/CV-8 biwire)
• Power amps (VAC 200iQ, Phison A2.120SE)
• Phono stages (Hagerman Trumpet MC, Herron VTPH-2A)

The consistent gear I haven't swapped out yet has been:

• Clearaudio Innovation Compact turntable, Universal arm
• Benz Wood SM cartridge 0.8mV
• Audio Research Reference 6 preamp

What do you think is most likely the cause, and how to fix it? I'm not real eager to "try" to reproduce the issue, for obvious reasons. But other than this issue, the rig sounds great - really enjoyable.