I am regularly a guitarist buy have been playing bass in my band due to the fact we needed one... I have been playing it through my randall g100 with decent results so far. I will be playing a gig in a couple weeks and am wondering if i should use my laney 4x12 cab in conjunction with my amp or if that will be a bad idea for the speakers... the ones in the amp have held up good so far.
It is widely accepted that playing a bass on a guitar rig will affect the cab, but not the head.

Guitar speakers aren't made to handle the low frequencies of a bass. If you like your cab I would advise against it.
"You can drink an ugly chick hot, but you can’t drink a fat chick skinny."

Fender: HSS Stratocaster

Modulus: 1991 Q5

Marshall: MG30FDX
Acoustic: B200
If you use loads of amp or guitar distortion you should be fine.
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Through a 412 I can't see it doing much damage to be honest. I used to run through a 410 and a 412, and the 412 was a guitar cab. I always liked the sound. It'll sound like straight ass and failure playing through a guitar head, but do what you've gotta do.