Re: bass cab speaker vs subwoofer?
Depends on exactly what you're talking about.
A 15" driver cone isn't a "subwoofer" by itself; it's a good start, but the tuning of the cabinet matters just as much.
Bass speakers, in general, are designed to be full-range. You'll see tweeter horns etc to emphasize higher frequencies, and bassists will play around with using combinations of 10", 12" and 15" speakers in full stacks or double stacks to fine-tune their response curve, but the frequency response curves of bass "woofers" tend to reproduce as much of the instrument's range as it physically possible.
The same is not true of subwoofer cones and the cabs they're loaded into. These speakers are designed for one thing; to emphasize bass frequencies starting around 250Hz and continuing down into the subaudible range. To do this, the cones are big, heavy, high-travel, and loaded into a tuned cabinet designed to resonate around 20-30Hz so the cabinet will help keep the overall response curve steady as the cone starts to bottom out.
You can load a subwoofer cone into a bass guitar cabinet, but the result will likely be the worst of both worlds; the subwoofer cone will sacrifice what high-end clarity a "full-range" speaker of that size would otherwise have, but the cabinet won't help it as it bottoms out, so the net result will be a cabinet that's good for maybe 50-500Hz and useless outside that range.
The first thing I'd check is, are you running the bass cab from your guitar amp? If so, invest in a Y-split and run "stereo" into a dedicated bass amp driving that 15". Most guitar amp circuits are tuned similarly to the cabinets to emphasize the frequency range of their intended instrument. If you have an instrument producing bass frequencies, you need a rig that expects to have to amplify them. Guitar rigs are expected to amplify frequencies beginning around the 70Hz range; bass frequencies are about half that.
Power handling of the stock B115 cabinet should be more than enough. I would however check that your guitar amp can handle the current draw of your guitar cab and this 8-ohm bass cab in parallel; if your guitar cab is an 8-ohm, total speaker impedance is 4 ohms which is as low as I've ever seen a guitar amp (though I've seen bass and PA amps handle 2 ohms). If your guitar amp is only rated for an 8-ohm minimum load, you're living on the edge, and very likely you're experiencing some "sag" as the low current is exhausting the charge from the capacitor bank.
