After a little crude experimentation, I slapped backs on three speaker cabinets I've recently built and, while I will probably make some refinements as I do more research, the bottom end of the amp has gotten much more pronounced. 

Advice:  Taken.

Results:  Excellent.

Thanks, J.M.  That's precisely the kind of sensible advice I was hoping to get here.  A closed-back cabinet sounds (hopefully literally) a million times better than going crazy with the soldering iron.

At the moment, I'm playing in a guitar/drums rock and roll duo so I need to compensate as much as possible for the absence of a bass guitar.

I'm playing a late-sixties Vox Berkeley III solid state amp and it sounds terrific and I love it. 

Except I want more bottom end.  Would it be realistic for either me--who has built a few guitar pedals--or a qualified amp tech to swap out some caps here and there and open up the bass? 

Or should I let this amp be what it is and look elsewhere for my fat bottom?