I'll check out the owners manual in a minute, but here's what comes to mind:
1. You'd need to have the receiver's bass management set up so that it would send bass to the sub when only the front speakers are being used. So, either have the front speakers set to "small" (or the equivalent), or have the bass output set so that it gets sent to both the front speakers and the sub.
2. The Pioneer might have a direct mode that bypasses the bass management, which would then send the full bandwidth to your front speakers and not use the sub. You'd want to not be in that mode, if it exists.
I'll check through the manual for this stuff, but wanted to get you started.
Adam