Hmm, my subs actually take a fair bit of volume to kick in, when I'm playing talk stuff it's barely audible and the bass only kicks in at higher volumes.

But I do have two amps, one for the speakers and another one for the subwoofers, and they have different levels set so that could be a happy coincidence.

Maybe try playing with the level and bass boost knobs? If you're lucky, you may land in a spot where the bass is less sensitive and gives you more treble at lower volumes. Lowering bass on the radio and adding more bass boost might achieve this, counterintuitively.

Otherwise, it might be easier to switch in a low pass filter on the input, which can be done with just a capacitor. That'd be much easier than trying to switch the entire systems with relays and whatnot.

ah, you're worried about the bass being overpowering in some voices.

Most subwoofer amplifiers have a +12v trigger wire that must be connected to the radio so that the amplifier knows when to turn "on". Otherwise it would drain your battery constantly. It's pretty simple to add a toggle or push button on/off switch inline with that trigger wire so that you can choose when you want it on or off.

most amps also have a configurable lowpass filter that lets you select the cutoff frequency for the sub, so it will not drive any frequencies above that. Generally setting it to 120hz or less will make it not react to most voice without needing to turn it on and off all the time. I run the 12" 400w sub in my old Honda around 90hz cutoff, as the speaker really can't effectively drive anything higher than that, and it leaves basically all vocals untouched.