The more subs the merrier?

OK, a question from the "dumb 'ol DJ" here.

I currently use Peavey PR10's on sticks powered by a PLX-2402 fed by
the high out of my active crossover, and a single 18" Low Rider fed
from the "mono sum sub out" powered by a bridged PLX-3402.

Obviously (I guess) 2 subs fed off the low out of the crossover in
stereo would be better than what I have now. My question is - how much

Nothing in audio is obvious.

better? I seem to have plenty of low end now and things sound good,
but will I gain anything sonically except higher output? Am I losing
anything, or how much by going with sum/mono?

Actually subs in mono in my opinion are better than stereo

It will lend almost nothing to Stereo separation. But increase your SPL.