Hi earwig,
I listen to electronic music alot and watch sci fi movies sometimes. Quite a bit of sub 20 hz information. I tried to use 2 and 3 ways with 8-15" bass speakers because I wanted to keep it simple. None of them played as low and clean as a dedicated sub made to play sub 20 hz.
The bonus is it cleans up the midrange AND gives that atmospheric "hall effect" to recordings, including for ex. Larry Coryell playing acoustic guitar into a mic.
I try not to get too deep into someone elses decision but you are asking for what I wanted, so I'm giving my experience with how I solved my situation.
What kept me away from a sub before was I thought it was complicated to set up and adjust. It isn't so difficult it just took me to learn to "think sub". I use mine now cutoff ~ 70 hz and it blends perfectly to my main speakers.
My room is 12' wide by 26.5' long. I get away with one sub against a wall just behind the right speaker (which is 6' out from the front wall). I plan on getting another sub this year.
If you decide to get one or two subs there are several people here who can help you with set up. It just takes a little time and effort adjusting controls and moving it around to get it right. Mine is powered and is great for me as it has everything onboard and the sound is tight, deep and high quality. It was easy to set up innitially, but i also learned to adjust it's placement and controls even better over the months that I've had it. Seperate amps and crossovers could of course work well, just depends on how involved you want to get. I do have room treatment including large corner bass traps that help keep the bass response smoother than without.
As to SP Tech speakers I know nothing about them except I have always wanted to hear them, but I could never afford them anyway. I completely believe thay play to below 30 hz, why not? I think the knowledge and skill put into them is as high as I've heard of. They may satisfy you too. I bet they sound beautiful.
Don