I bought 4 subwoofers and I'm absolutely not doing a DBA! Hah!

You probably know this but just in case and just a suggestion: check with the driver manufacturer’s specs on the cabinet’s internal volume? 

@iseland "Plateamp": I moved my "plate amp'' out of the sub box into a separate housing. Its still a plate amp (components mounted on a plate), just a 'plate am in a different housing'.

Given the same space for the sub, I wanted that extra volume for the driver, and it allows me to swap out amps/crossovers, or use the 'housing amp' to drive other subs. 

Its the 2nd post I read today that mentions "enlarged soundstage"
and "air" in the context of woofer/subwoofer ("2 subs provide enlarge soundstage and more air"). 

I never brought the low frequencies in context with 'air' and 'soundstage'. 

Should I start thinking adding a 2nd sub (2 smaller ones instead of 1 larger)?

Me personally,  there are plenty of excellent amps without plates in already made boxes I would buy instead of making my own.

And don't get me wrong,  I love my hypex plate amp ! But if I  didn't had the need for convenient space saving solution I would buy a typical all in 1 already made box .

@kraftwerkturbo It's a real thing.  I suspect much has to do with the appropriate high pass filter though.  Several a'goners who I have convinced to try raising the HP point from 40 to 80 Hz or higher have reported the improvements along those lines.

I have had this happen with 1 sub.  It's not about how many subs but about the sub to main speaker integration, but if you want to add subs, who am I to stop you?