I’ve got an 18-inch subwoofer, some mids, a couple of tweeters, and coaxials in the back. Would adding a dedicated woofer or midbass be useful?
Wait, what do you currently have set up, and what are you hoping to improve?
You could test it using pink noise and a spectrum analyzer. It really depends on your speakers, settings, and the environment. But my guess is that the 18-inch sub is focused on the low end, so adding some midbass could help fill in the mids or at least let the sub focus on the lows.
If you’re missing midbass, then yeah, go for it.
But most mids can handle down to 80Hz… unless you’ve got those sealed basket mids that are only for mid frequencies.
If you enjoy electric guitar music, then yes, a dedicated woofer might be a good choice.
If you’re using an 18-inch sub, I’d suggest adding a midbass and setting the sub’s low pass filter around 60Hz.