I've got something bookmarked somewhere-I'll see if I can dig it out.
FWIW I bought a cheap but originally quite expensive big car one off my GF's nephew & used that. I'm using the LF out from my AV into an old 60W technics hifi amp. It's behind the sofa & seems good enough to me! Inphase is what's written on it.
There's an awful lot of air moving around on subs. I'm not, initially, quite seeing how you'd get away without some porting.