12-inch Speaker options?

It's really just suited as a guitar cabinet.
If stock speaker sucks then a generic V30 or Eminence Cannabis Rex.

Open back guitar speakers though.
For bass just run external cabinet.

Any bass worthy 12" isn't designed for open back. Beta12a2 is decent driver for low watt bass amp. Plenty opmmmhh for 15 watter.
Not designed for open back regardless

Eminence Alpha 15 won't fit. But it has interesting specs that make it feasible open back driver. Often used by hi fi guys for open back projects. Cause of it's unique specs.

 

If you seal the back how are you going to handle the cooling that the tubes require?

Edit: In my mind use the amp as it was intended. Trying to make something into something it’s not usually ends up in a failure.

 

The trick would be to build a new cabinet. You could give it an open section for the amp, so it has all the cooling it needs, and a closed or ported as you choose section for the speaker. The original cab and speaker get stowed away in the loft or whatever. Then when its time to move it on (or if you decide the combination hasn't worked out) you simply move the power section back to the original cabinet and its 100% stock again.

 

Extension cabinet.

 

That cab is on the small size for a 12 inch bass driver (about 1.25 cf).

The cab can't be sealed because the air circulation is critical to the cooling of the tubes.

While the amp is a tube amp, it is only a 15w tube amp. Even if we outrageously pro-rate it as 60w (4x), any 12 inch bass guitar speaker will handle everything this amp can dish out.

A solution that is reversible, easy and will yield 'pleasant' results is to use a high Qts 12 inch driver in an open back cabinet. the driver would have to have a response with HF extension to 3-5 kHz, and Fs below 60 Hz. Consider this one: "Goldwood GW-212/4 12" OEM Woofer 4 Ohm" from www.parts-express.com!. No cab mods will be required. This will yield a nice bass practice amp. Nothing more.

OTOH, this is a nice guitar amp. If you really want a nice low power tube rig, consider an Ampeg PF20t into a compact 1x10 cab.

 

Kinda tough sealing the back.
Well actually it's easy but....
Got tubes back there

Any typical bass 10" does fine in .9 to 1.3 cubic feet. Just simple eminence beta 10 would handle the wattage. Ironically the beta 10cx coxial driver does really well in very small enclosures. Just plug the tweet port.

Technically don't matter open back

 

The alpha12A would be the best thay could offer in a 12 inch for open back cab use. It has a reasonably low Fs, highish Qts and a bass guitar friendly frequency response. It has a higher sensitivity than the Goldwood GW212/4 (by around 3 db), but will require more bass boost to get a balanced tone (due to the lower Qts). Given the limited power available, the two drivers will likely produce about the same maximum volume.

Purchasing or building a separate bass cab to use with the GVT15-112 will net you the best results with respect to volume, bass tone and guitar tone, but at a cost of bulk, weight and money. A open back cab is easy to make - a 2cf open back cab with the Alpha 12A or the Goldwood, would sound a lot better than the same drivers installed in the Ampeg cabinet. A 1 cf ported 110 bass cab (like Dukes Coffee House 110 DIY) would sound better still, and would be smaller and lighter.

Decisions, decisions...

 

I replaced the stock speaker of an Ibanez TSA15 with an Eminence Alpha12A. That gave some improvement in bass response. But a separete cab sounds better. For a bedroom/living room practice amp it is oke.

 

I know in hifi circles people like using the alpha 15 in open baffle projects. Could be worth looking into if you want a cheap open back extension cab. Granted, if I were building a cab I'd be more inclined to more efficient designs. A horn would be most efficient, but is the hardest to build and design, but a sealed back cabinet would provide a great step up and wouldn't take much design and work over an open back one.

Granted, as a drop-in, an alpha 12 would probably provide the best cost and design/work to results ratio for a 15 watt amp over any extension cabinet.

 

I once played the GVT-15 head and found it way more suited to guitar than bass. Sounds good on guitar, though.