Two easy fixes. A simple wooden toothpick to wedge the baseboard or a spot of glue that's allowed to dry. If the baseboard needs to be removed, simply cutting the glue joint with a razor blade, without damage, will take care of the baseboard removal.
I think there's a case of resonance as opposed to a need to separate the sub from the floor. When I do frequency sweeps, everything from the subwoofer, to glass panes in curio cabinets to the grill on the fireplace rattle. Nothing that a few well placed toothpicks won't fix.
For isolation purposes, I'll use anything conveniently at hand, whether bubble wrap, rubber grommets or carpet pad.