Well then here we go…
Last night I went ahead and reran my experiment (there, does that word make you guys more comfortable?). I kept the right V3 Mono connected via balanced input and speaker cable connected, “in the normal manner,” so red to red and black to black at the amp and at the speaker. With the left channel, I initiated the experiment by changing out the balanced interconnect for a single-ended one by disconnecting my subwoofer (which is normally connected with a full stereo signal from my preamp’s single-ended output) and using the left channel RCA cable to connect the left V3 Mono and flipping that amplifier to RCA input. I turned on both amplifiers and played pink noise on loop, and level matched the left and right speakers as close as I possibly can from within 5mm in front of each driver (they’re coaxial, KEF LS50 Metas) using the balance plugin on my streamer (granularity level is only 0.5dB—apologies that I cannot get finer balance control than this, as my preamp does not have a separate balance control). Initially, I ran into a strange discovery in that if I use my preamp in any other setting than Passive preamp (no buffer) mode, I can max out the balance plugin to the left side and the right side would still be too loud, but that’s a separate topic all together. Anyhow, using passive pre mode did the trick. Then I sat back at my listening position and switched to music. Lo and behold, the left and right channels *TRIGGER WARNING* sounded clearly out of phase with each other. Finally, as the last step of the experiment, I shut off the left V3 Mono, flipped the cable connections at the amplifier end so that speaker had red to red and black to black, but connection was now red to black and black to red, and turned it back on, and now both amplifiers were back in phase and sounded as the system is supposed to sound (when I am not using a subwoofer).
There you go, end of experiment. After that I reverted my system to normal configuration and moved on. If this experiment is not good enough for you, feel free to ignore my post. I was asked to do this and post back by members other than you.
-Ed