What does a subwoofer solve?

My previous amps were BHK 300 mono blocks.  The bass they provided for my Monitor Audio PL 300ii speakers was exceptional.  For several reasons I replaced the BHKs with a Luxman M900u.  Very happy with the new amp, except for disappointing bass performance.  Would a set of good subwoofers help resolve this, or do subwoofers only address a lack of bass in standard speakers?

Thanks in advance for any advice or suggestions.