You need something that isolates the grounded RCA's from the Bose capacitor input stage, while still allowing regular signals to pass through i.e. the capacitor cure in the link. All I can see on "floating ground adapters" is they provide a line out converter from floating ground speaker outputs, to floating ground line outs i.e. a regular hi-lo or LOC, which was the other fix explained above, with all the problems associated with doing it this way.
Did find this for 8 bucks (in the link, no idea where your "techs" get $60 from.

• http://www.stompaudio.com/rca-filter-floating-ground-adapter-p-8736.html