#1 clear white pine is about $5 a liner foot here. It's supposed to be stable.
You're probably looking at #2 with some knots. If it's straight and flat, a rare combination, it would be ok for a guitar cabinet.
For a serious bass cabinet I'd stick with a high grade of 3/4" plywood. 11 1/4" isn't really deep enough for a bass cabinet.
If you can get the stuff free, I'd try it out a while before investing the time and money in covering it. It would surely be fine to make some experimental boxes.
Yellow pine is very heavy, but I doubt if that's what you have.