I recently bought a REL T-5 subwoofer. It's a small guy (12" cube), with an 8" woofer and a smallish plate amplifier. REL does a little different take on subwoofer connections. Their preferred method is not to use an LFE input, but rather (with supplied cable and Speakon connector, at one end), would rather their customers connect the sub to the speakers outputs (those amps that have common ground) which almost all are today, at least Frank's are.
The input impedance of the T-5 is around 170,000 ohms, so the amp will not even know the sub is connected. What this means is that the mains are connected to the amp as normal, then the cable going to the REL is connected with 3 wires, one for left channel, one for right channel and common at the amp, and then connects to their sub with the speakon.
Then dial the cross-over way down and let the sub continue where the mains left off. It blends really well.
So I also have an old ARS300 (Acoustic Research with a Carver Sunfire amp plate), and it also has the more traditional "high level" inputs. I hooked this amp up the same way, running wires to my mains in the regular fashion, and then ran wires for left and right from the amp to the sub. Same kind of results. Seemless integration. I don't know why I didn't see the light before, But now I think all subs should be hooked up this way.
Wayner