With the VC, the 300 ohm tap is often unlistenable due to noise, regardless of the tubes used. It’s interesting how others have success - lucky them.
Generally, for the ZMF VC, I use the 32 for instrumental, and the 8 for vocals where vocals on the 32 can be too shouty and inappropriately placed among the instruments (and 300 when I have no regard for noise and want the driver to be fully unleashed and my world to be rocked while on stage). Choosing the lower impedance results in the amp exerting more control of the headphone driver, which can tighten the bass and attenuate sonic properties. Whether that is favorable depends on the listener’s desired presentation (and varies by system, apparently, given the inconsistency of 300 results/usage by various listeners).