Pretty sure you haven't told us what cabinets you're using now, so no idea if they'll be louder. A high pass can provide more output by limiting the pass band but it's main advantage is protecting loudspeakers from mechanical or thermal failure. They are often found on the same devices providing the low pass filter. If they're not you might consider adding a crossover that does. I would strongly recommend reading up on high pass filters before your first gig.
Unless I don't understand you, those numbers don't align with each other (more like 20 Hz tuning) but in any case they're big cabinets with a low tuning (very low if you consider their use). The reflex will have a very lounge-ish deep bass and output, which sounds like the wrong tool for the job. The Cubo 18 focusses on concert level output from 40 Hz and up (much louder above 40 - 50 Hz). So for concerts Cubo 18, at home I would rather have the reflex.
Thank you. I started doing this 3 years ago because my cousin needed a sound man for his band. I had zero experience and research was tedious for me. I had no idea that i would love it so much. Your website is the best resource ive found. Thanks again for your time and for helping me.