Pete - lower priced entry level acoustic preamps are readily available..

Check the products stocked by the big UK internet music gear shops,
or Amazon & Thomann...


I don't know if Bosss still manufacture the AD series acoustic preamps, or if discontinued...???
10 to 15 years ago I used a Boss AD3 to try to tame a very unpleasant sounding piezo mandolin, which worked ok enough,
and not too pricey second hand these days off off ebay.

Or maybe check ebay for the occasionally sold Award Session Gordon Giltrap pre amp. which was well respected in it's day...