Any educated ideas about this?

[Arty]

would be fun to offset gravity with DC

 

[Shadydave]

OB Compression

would be fun to offset gravity with DC

Great, offset bias compression.
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[Inductor]

why not just not re-invent the wheel, and..
since You are in hungary, just order:

http://hangszoro.net/adat/421.pdf

2 of these, they are decent.
price is under 8K huf, and bloody sure with room gain it will give You deep enough bass.
93dBL sensitivity makes sure it is dynamic. I remeber these drivers as fast subs, just what You need.

Those drivers have good LF extension with F10-20Hz/F15-15Hz in a VB = 132L BR enclosure. Behavior is not so good for a 80L sealed, let's say, F3=48 Hz. A small 10W amp is enough for max. excursion, if I'm not wrong. (edit) Yes, I checked again: 6.4 W@3.5 Ohms/20.6 Hz for 3.5mm Xmax, Xmech being 6.0 mm, maybe not needing a high pass in a current home environment with infrasound quality.

More on the H2000 Speakers (Videoton) - Hungary
Mélyközép-hangszórók
HA31-12" Series:
HA31-421 Mélyközép-hangszórók : HA31-421
HA31-422 Mélyközép-hangszórók : HA31-422
HA31-422G Mélyközép-hangszórók : HA31-422G
Price in Euros is between 27 €/38 €, you can't get wrong for a "pro" 12" driver.
Some history: http://www.diyaudio.com/forums/multi-way/224668-new-h2000-midwoofers.html