Need advice on adding subs

There is a good thread currently running elsewhere on the site, under the simple title "Subwoofers" (I think). Duke of AudioKinesis and BDP24, among others, have made some valuable contributions, and it is worth reading. Here’s the link: https://forum.audiogon.com/discussions/subwoofer?highlight=subwoofers

It's a long thread, but worth the time to read through- some thoughtful stuff that I benefitted from. 

cjk: I think the take away re paper v aluminum is that the paper will sound more natural if you are solely using it for music and/or want to x over higher. I bought one that was delivered Friday as part of an experiment- a paper coned 12 in the ebony gloss finish using the xlr’d version amp.
The takeaway, as far as I’m concerned--others may have different views- is that multiple woofers even out the room response.
I have experimented with subwoofers for decades in my two channel audio and was never happy- trying to match them with Quad electrostats. Not only have the subwoofers improved, i think in part due to home theatre and car audio (JL anybody?), but it seems there is renewed interest in and thinking about high quality sub-woof’ing for two channel audio. A lot of what Duke wrote in that thread made sense to me in terms of "apparent" speed- sensitivity to frequency and volume, rather than just timing (if I’m remembering it right, it wasn’t leading edge timing that’s the issue, but ’stopping,’ among other things).
I’m also experimenting with DSP and a bigger old woofer- I have a bunch of Velodynes from the old days- an 18" (size of a coffin) and a 15"- both are well past their shelf life, but I DSP’d the 15" and am x’ing it over very low~ 45 hz. The Rhythmik is running without DSP, placed midwall on the opposite side in a corner-and crossed over a little higher-it’s a weird room. I got the levels matched at the listening position and have only enough loudness that they blend into the multiple woofers in my horn array. With some fiddling around-a few crude measurements, and some listening, moving, and listening, they seem to gel nicely and add a nice further dimension to my listening. I don’t listen particularly loud to begin with, but augmenting the bass--running as a parallel system, no high pass to my main system--has given the system more gravity and spaciousness, more music at relatively sane levels.
The moral, I think, is that these things can work, not just because the technology is better than it was 20 years ago, but because the strategies for placement and dealing with room interactions are more effective.
PS: I can't speak to the open baffle or dipole, one of the others can address. I know that using a dipole like a Maggie was one thing HP used to tout with the Crosby Quad back in the day, but I never tried it. A DIY build makes sense to me even if you have to get some help from a competent cabinet maker. Good luck and have fun. I certainly am, and it has added joy to an already good system.