Subwoofer suggestion

Svs SB 4000, free delivery around $1800 nice app. control it 45 day audition 

5 year transferrable warranty, it matters not how loud you play it’s the power reserves and Quality of Bass , and you can play it wireless with a kit .

While it’s not an apples to apples comparison, I recently replaced a 30 year old Paradigm PS1000 with a very nice used B&W ASW855 for my second family room system.  I have an SVS PC12 in my main system which performs respectably, but this B&W is mighty impressive. It is effective down to 15hz and seems to outperform the SVS in most, if not all, aspects. If you can find a good used ASW 8 series it may do well with your other B&W speakers.  

Sidenote: using 2 subs was mentioned as beneficial. Assuming same 'air movement capability' and budget (think 2x 12" vs 1x 18"), what ARE the perceived noticeable benefits? For discussion sake, I counter that the single LARGE sub will reach lower and is easier to 'hide' than 2 small subs. 

@kraftwerkturbo 

2 subs will help mitigate room modes in the bass region better. 

Would 2 subs placed close to the main speakers also allow higher low pass setting? Thinking using very small satelites that struggle at higher output levels and 80 Hz.