Hi Brian,
There are a couple of things going on here. If you are listening at low volume, increase the Pioneer's sub output level in the Pioneer menu so that there's enough voltage to "wake up" the EP350.
But at low volume especially, human ears are very insensitive to deep bass, so to hear it you may have to manually increase the sub's volume (using its own level control) to compensate for your hearing's relative insensitivity to deep bass at low levels. Whether or not it's worth the bother is up to you.
Years ago, some receivers and preamps (Yamaha in particular) included a variable "loudness" control in addition to the conventional volume control. You'd preset the volume at a normally loud level, then turn the loudness control down to your preferred low level. It would automatically boost bass by increasing amounts to compensate for our reduced sensitivity. It worked fairly well but most consumers didn't understands its utility and were confused by two controls, one labeled "volume" and the other, "loudness". So you rarely find it these days on new receivers or preamp/processors.
Regards,
