The problem with variable phase dials is that AFAIK nobody has ever made a "variable phase" circuit, it's a "variable delay" labelled as variable phase.

Ideally I guess you would have both; a switch to invert and introduce a 180 degree phase shift in all signals no matter the frequency, and a variable delay for fine tuning. Then again, since AV receivers frequently have the variable delay covered...

... (light slowly flickers on) I guess that's why the switch is standard these days ;\)

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8