I seem to have confused the terminology between signal and power planes! There is a ground plane, plus potentially four power planes -- +/- 15V, +HV (200V), and i'm not sure if a -VREF (~ -7V) reference voltage should be considered a "power" or a "signal".
I guess what I'm really wondering is whether it's considered kosher to turn the +/-15V, +HV, and -VREF into solid copper layers analogous to the ground plane, as opposed to trying to draw out all those traces.
I'm laying out a schematic for a piezo driver on a multi-layer PCB. The schematic has potentially five signal layers -- a ground, +15V, -15V, -VREF (-6.95V), and a +HV (+200V). Is there any advantage/disadvantage to having 5 signal planes? Is there a "best practice" on how to order them? The output is a ~0-150V signal, which might be a low-frequency sweep of ~ 10 to 100Hz, or a high(er) frequency dither of ~ 1-100kHz.