Bag End S15. Heck, they even market it to bass and guitar players. I've gigged mine with both instruments for the last decade, and my dad's been running an S15/S12 stack for guitar and bass for 35 years.
By me, you can find '70s Fender Bassman 2x15" cabs for a couple hundred bucks. They do pretty well for both, if you have the space for a big cab and don't overpower it on bass (I'd run no more than a 200 watt head and try not to go below low D). I bought mine from a guitarist, gigged it with bass, and sold it to another bassist. Then bought it back, gigged it some more and sold it to a guitarist. Then bought it back again, and sold it to a bassist.
By me, you can find '70s Fender Bassman 2x15" cabs for a couple hundred bucks. They do pretty well for both, if you have the space for a big cab and don't overpower it on bass (I'd run no more than a 200 watt head and try not to go below low D). I bought mine from a guitarist, gigged it with bass, and sold it to another bassist. Then bought it back, gigged it some more and sold it to a guitarist. Then bought it back again, and sold it to a bassist.