I have the OEM Pioneer T-1809 head unit. Subwoofer volume adjustment is a major pain, particularly when driving. Insult to injury, subwoofer frequency adjustment requires dialing to a different screen. It's hard to find a happy medium so I installed a remote volume/ level controller. Now I can adjust acoustic bass to metal without having to touch the head unit.

The remote sacrifices a little volume so I maxed out my amplifier adjustments then throttle it with the remote. The remote frequency ranges from 40hz to 150hz. The volume ranges from -12dB to 0.

Found it on ebay for less than $13 w/ free shipping. Probably available on Amazon. You'll likely need a PRIME membership to get free shipping.