Slightly OT: Does anyone know how to add a tweeter adjust to a cab?

Ya you can, just add a crossover, single pole, a 2.2UF/80v in series with a 4-30 Ohm 10 watt resistor depending on the loudness you want from your tweeter. Connect that to the + of the tweeter and the - connect to your woofer -.

 

Sounds like what you're asking for is a thing called an "L-pad"

It's a potentiometer, but it attenuates the tweeter without changing the impedance that the crossover sees. After all, if the tweeter is 8 ohms, it needs to stay 8 ohms, just attenuated.

Fixed attenuation circuits add a resistor in series plus one in parallel to keep impedance proper. But a fixed circuit depends on you knowing exactly how much attenuation you want--3 dB? 6 dB?? and then you figure it from there.

But if you don't know how much attenuation you'll need, or if you may have to vary it, you buy an "L-pad" for a few bucks. You don't need a massive one, since tweeters ordinarily don't see much power. Check Parts Express www.partsexpress.com for different prices, depending on how much power the tweeter will handle.