>>but if I either move the volume knob a little bit higher or the gain at the preamp or the volume at the preamp, then the sub does not respond until I cross a certain threshold, and then it increases the volume not evenly

Yeah, that sure doesn't sound right. Glad you have a new sub on the way... let us know how it works out.

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