absolutely not!
you have all you need with the XR18 and powered speakers. it's got way more than enough tricky processing.
a real sub will always make the difference in how good it sounds out front. the sub handles the lows and lets you take the lows out of the top cabs, so they sound louder and better too.
either way, some XR18 basics:
go to "routing" and set your aux outputs to be either "input" or "analog", so that changing them in the mains won't affect them in the monitors or vice versa.
kick in the high-pass filters on everything except the kick, bass, and maybe the big toms, set to about 100Hz.
don't compress the overall mix but do compress the vocals a little bit.
use the parametric EQs for everything; they're so much better than graphic EQ, and with a touchscreen they're finally easier to use too.
do what you want on channel EQs (cut lows on vocals, boost lows and scoop low mids on kick drum, leave bass guitar mostly flat) but on the main and monitor EQs, only cut, never boost.
a pinch of reverb on the drums (and other "acoustic" instruments like acoustic guitars or horns) and a bit of delay on vocals.
finally, if you're an engineering type, get yourself the yamaha sound reinforcement handbook and study it, many questions will be answered.