DBX made a downward firing 15", subwoofer using a niobium, rare earth magnet.
"I think it was a, DBX model, (DB-15)".
It came with a separate amplifier and uses the "Speaker Level, Line-output" from your amp to the Rt./Lt., as the inputs are marked on the DBX amplifier.
The amp states (150 watts). But I swear it hits like my "Martin Logan Descent" subs do with their huge plate amps!
So, with the DBX, you use speaker cable from the speaker outputs on an "integrated", (Usually called the "B Speaker" outputs). Or the same, "speaker cabling" from your component, amplifiers binding posts to the amp.
The (Rt. and Lt.) speaker cables are both used from your amp and then into the "DBX" amp. Which has an analog "Output Level" adjustment pot/dial, on the front of the amp as well.
I do still have it, in case you or someone else happens to be interested...
And as a for the price? "But for a song and a dance"! The MSRP was about ($800). I believe "From Crutchfield", purchased new back in the mid 1990's.