If I select the MaxxBass filter instead of "audio" or "original bass", then it's just letting the harmonics through right?
That's just for monitoring purposes, those are monitoring buttons. Reads to me like you're trying to use MaxxBass for a purpose that is was not intended. Original Bass lets you hear your original bass without MaxxBass harmonics, MaxxBass lets you hear what the plugin is doing to the bass only and Audio lets you hear the whole mix with MaxxBass harmonics added.
Ideally you would want to use MaxxBass on the master fader/buss. Throw a maximizer after it to get your level back up. The only thing I would put before MaxxBass is an EQ...but only if needed.
EQ - MaxxBass -L1/L2/L3 or just MaxxBass - L1/L2/L3
p.s. let me add this. If you have good monitors, say with at least 8" drivers, when you add MaxxBass you won't hear a big difference. You'll only notice a difference on smaller speakers. Ones that generally can't handle low frequencies very well. Using MaxxBass or RBass will actually make very small speakers "boom" with clarity where as if you didn't use it they would sound like shit! That's what MaxxBass & RBass are designed to do.