Edit: We’ll be playing techno, dub and deep house.

Remember plans are for internal measurements and using 19mm ply, change to 18mm and add some bracing.

https://forum.speakerplans.com/bc-sub-18s_topic45717_page1.html" rel="nofollow - B&C sub 18's - Speakerplans.com Forums - Page 1  Thread here on 186 in sub18. 

Search around more for some build pics for bracing and construction.

Does the OP have original PD186 or later version !?

Not sure if later versions of PD186, will work as well, in same cabs, as the original.

If you did want to build more boxes to match in future and use the system outside or in a larger venue, then 184 could be an option. Although some might say that due to the 184 design being over 20 years old you might be better off looking at something else. The answer is as always; it depends! 

186 would be even worse in small room than 184 in my opinion, will not drop low at all and designed to be used in block of 4 or more. My experience of messing around different subs in my small 11ft by 21ft studio is that anything other than reflex sounds pants. 

If you want to build something more 'exotic' than reflex for the experience/looks/building a larger system in the future and don't mind that it wont sound great, then do it. If you want what will sound loudest and lower in a small space then go reflex.

As mentioned above Mk1 PD186 sounds great in the sub18 plan and would provide more than enough bass to shake your room.

https://forum.speakerplans.com/pd-186-xmax_topic38201.html" rel="nofollow - https://forum.speakerplans.com/pd-186-xmax_topic38201.html

White dot on speaker - Ear defenders & look maybe!

As thread xmax is approx 10mm

So 18 sub reflex cab driven to 10mm (grey)and Martinson Roar 18 driven to about the same excursion. Using old 186 parameters I had. Not confident that the 186  would be a good match to the Roar18, it might break, but was looking for solutions that might give you that bit more!

I should add don't test with a constant tone for more than a few seconds & away from the excursion minimum so probably 60hz plus - To get 800w from your CS800 you must be running in bridge mono?

The reloop box you have looks an active sub, have you removed the amp module and made it passive for use with your peavey amp? Is the 186 what originally came in the box?

Ms18 mk3 would give you more output than a reflex so could well be worth a try, but again in a small room I would imagine you would get a lot rattling of stuff in the room but not much sense of physical/musical bass in there. I found that 2x15 reflex 'feels' much louder than 2x 18 mini scoops in a small room.

Other things to think about are placement of the sub in room, placement/crossover points of tops, time aligning and eqing to suit the space.  

Maybe as a test try using the mixer on the DSP out and then into the Peavey to see if you can get enough drive signal? 

Looked for info on the MiniDSP output level but not found anything other than its supposedly balanced TRS jack out.

Second, the room is always a big factor. To gain the most from your sub, put it in a corner. Reading your other posts, I doubt a different design will fix your problem.

So building another 18 could solve our problem? I’ll play around with placement and see if theres a decent place to put it. I heard about putting the sub in the listening position, and then put it at the spot it sounds best. Does this work? If so I’ll go around with the db meter and put it where it’s loudest

He will reach that 100dBZ with ease, so I should have probably made my statement a bit more clear.

Second, talking just one cabinet, the problem will stay. Multiple smaller subs with correct processing is a different story as you described.