Add a sub (or subs) or try an amp with a very low damping factor.
If the speaker cables are 12’ (or less) try single runs of 22-26 gauge solid core copper wire as speaker cables (doing so should bloom the bass a bit).
On the cheap try basic copper magnet wire and if that works look into Neotech 22-26 gauge solid core copper/Teflon wire (still inexpensive).
Do not use wire larger than 22 gauge as this will defeat the purpose.
I’m suggesting a $10 experiment (with magnet wire) and I have been using 26 gauge solid core copper speaker cables for 20+ years with excellent results.
DeKay