Good for you, Cromdubh!

A double bass would not only be a huge additional expense, but would be exceedingly bulky and unweildy in a crowded pub environment. And I think you're right about those acoustic bass guitars (although there are one or two acoustic bass players hereabouts who would argue that point).

You seem to have the right approach: ask first, and be discreet with your amp, in terms of both volume and visibility. The purest of the purists will tell you no thanks, but you can still listen and learn at their sessions. Plenty of other musicians will give you a chance; assuming that you exhibit a degree of taste and discretion, they'll welcome you back.