Re: Speaker suggestions please
True Bass speakers tend sound sterile for guitar.
I have a Marshall 1x12 combo that I built a cosmetically matching closed back extension cab to stack on top of. I just put the stock V30 speaker into the bottom because it's brighter than another warmer sounding V30 that I put into the combo. The combo stack has more low end and a wider sound stage than the combo alone, which emphasied mids. The bass is dramatically improved, not quite up to my Marshall 4x12, but close.
Do you plan on using this speaker in conjunction with the combo's speaker, or as a stand alone cab? If you plan on using it with the other speaker all the time, then I agree with the G12H30 suggestion(30 watt power handling speaker). The heritage version of the G12H30 uses the cone (444) with the lower resonate frequancy, once broken in.
The Emi eqivilant is the Wizard.
What speaker does the combo have? The speaker you put in the extension cab will need to at least match the effiency of the open back combo speaker, or it won't be heard. If the extension cab is closed, then it would be prefered if that speaker has a higher efficiency rating than it's open back mate. Bass speakers tend to have lower efficiency than guitar speakers.