@olfac87 Hi, I'm a long-time lurker here. Your post prompted me to respond as I went through a similar thought-process recently. I have owned a pair of B&W 801 Series 2 since the early 90s. These are fantastic speakers. Those who say B&W speakers are bright have not heard these speakers, the last ones designed by John Bowers. Your father chose well: I suspect you may have the first iteration of those Matrix speakers, the 801 Series 80.
The 801 Matrix works really well in solid rooms. I had them in three houses that had solid brick walls and concrete floors and never felt the need for a subwoofer. However, when we moved to Northern Ireland into a property built in 1865, the basement room that is now my dedicated "music room" had a dance floor suspended on joists, with parquet flooring installed by the previous owners. The 50cm thick main walls were covered by plaster board to for a clean look.
In that room I never could get the 801s to sing, the last octave of bass had gone missing. I tried everything I could think of: I had my Krell KSA-250 recapped. I upgraded the XOs on the 801s. I bought a better preamp. I moved the speakers around all over the place. All to no avail: low bass was still AWOL.
At that point I realised that perhaps it wasn't my equipment but the room that might be causing the issue. Perhaps the plasterboard walls just absorbed bass below 40hz. So I lugged one of the dual BK XLSS-400DF subs from our living room system downstairs and hooked it up. That was quite a bit better. Then the other one came downstairs as well. After a bit of fiddling with the set-up of the subs I had a Eureka moment: wow! The bass was back.
Two new subs were ordered and delivered last month and after switching from using the line level inputs to high level/speaker inputs the system now sings.
Integrating two subs in a two channel system is not entirely straight forward but it's not rocket science, either. With full-range mains like the 801s it is best to think of the subs' crossover frequency control and the gain as a pair (or more accurately, a quartet) of parametric equaliser curves that you can adjust independently of each other. The ability to position two subs in non-mirror imaged positions also gives you more flexibility to tune them to the room requirements. To give you an idea: the subs' crossover frequency is set at 45hz which means that they only start augmenting the bass from about 35hz on down. The best way to judge whether the crossover frequency is set correctly is to listen to male voices: if someone sounds unnatural/speaks with a somewhat boomy voice, the crossover is probably set at too high a frequency. If you can hear a sub over the main speakers, the gain is too high.
I had an audiophile friend over yesterday. When I invited him, I only said, "I've made some changes. I think I've got it sorted now. Let me know what you think." He came down, sat down, and we listend. He started smiling as Hotel California started spinning on the Funk Firm Vector V. He looked around the room and said, "ah, I see - new sub!". "Not quite", I replied, "two new subs." He hadn't noticed - or heard - that there were two subs playing (one is out of sight from the listening position). "That's the best I've ever heard your system sound", he said. "It's true what they say about a good sub installation: not only do you improve the bass but you also get more transparency and resolution at higher frequencies. That 2M Black just sounds amazing." I was quite chuffed when I heard that.
@axpert's recommendation of REL is a good one. High level inputs do make the seamless integration of multiple subs easier. Alternatively, give Tom at BK a call to check about shipping to the US. REL used to subcontract the assembly of their early subs out to BK. The company still constructs speakers on an OEM basis for several well-known UK manufacturers. The built quality is exemplary, the veneer looks classy, and special orders are easily facilitated: my new mahogany XLS400s came with an auto-on/off circuit added at my request. I have no connection to the firm other than being a happy customer.