Welp I’m now 100% certain this case has a very simple explanation: power amp clipping. The 2 amps I’d been using were very similar in power: 100 Watts/ch tubes and 120 Watts / ch SS. Swapping in the 250 Watts/ch Rogue Apollo Dark, and it sails through everything. IIRC the Rogue also has much more dynamic/transient power reserves than usual (Rogue has previously mentioned 800 - 900 Watts?!), which applies here
I’d started to notice the issue was happening on certain spots of certain tracks. And yes, it was related to volume - it would always play clean at lower volumes. I just didn’t expect amp clipping to manifest a singular POP at so much of a higher level than any music the tweeter was playing. No grungy, hazy distortion or harshness or any other warning signs - just a hugely loud single POP occurring within otherwise perfectly CLEAN music playback.
So my educated guess is there’s an awful lot of subsonic low frequency energy getting into the system, even with good rack & isolation platform. From record warps (etc) and feedback. Yes, the tonearm & cartridge compliance is in "good" range. Unfortunately it’s a small room with a very bouncy suspended floor. So I suspect a huge LF waveform (that I can’t even fully hear) pushes the amp too hard, and boom - now it’s a clipped waveform with both a LF and HF component (the not-so-neat part of clipping a LF waveform). The HF component is very transient, but at a much higher level than all combined HF content from the actual music (because energy content in actual music decreases as you raise frequency), and of course it sounds just awful (edge of a square wave). It really did sound like a mega-loud singular surface POP. Weird that it usually seemed to come from the right side, even once when I had the MONO switch engaged (can’t figure that out). Tannoy’s tweeter is tough, thankfully - all OK there.
Anyways I think it’s best to have more power on tap to keep the LF crap from clipping and splitting into LF & HF crap. Long term I need to experiment with rack position and isolation tweaks to reduce feedback. Maybe even bass traps etc. Yes I’ll keep a lid on the volume, too. I know I play loud, but it really shouldn’t be clipping at these levels for just music without the (suspected) LF crap going on.
Many thanks everyone for your contributions, it helped me think through all possibilities :) The system sounds phenomenal now, btw. Very happy and glad to have an answer.
I went through all possible avenues, and the following definitely had zero effect:
- Anti-static measures
- Audioquest Niagara 3000 power conditioner
- Different cartridges (tried Benz Wood, Ortofon A90, Shelter Harmony)
- Different preamps
What’s interesting is I never reproduced the issue when using my VAC phono stage with EAR SUT (both borrowed from main rig). But the problem occurred with both Hagerman Trumpet MC and Herron VTPH-2A. My guess is the VAC + SUT simply filters out more of that LF energy. The Herron and Hagerman both use a JFET MC stage.