The issue is half in the speaker, half in the amp. Lots of speakers have challenging loads in the bass, and lots of amps can perform better with better power supplies. Notice this is kind of statistical. I say "lots" and "most." So, while bi-amping may help, it won't always.
When you bi-amp you double the power supply and output transistors, therefore making the amp less sensitive to impedance issues.
Whether or not it helps you is something you should hear for yourself. Generally you should hear better defined bass and transients. If you don't hear that you may as well sell an amp.