I have an Arcam A85 integrated amp. It has mains in/out on it so I run the line level signal out to a Paradigm X-30 active crossover. This lets me high-pass the signal at either 50hz, 80hz or 120hz (only those values). It also lets me low-pass the signal in that same range but it is variable for the low-pass. I then send the high-pass back to the Arcam to be amplified for my mains (I would probably use 80hz or 120hz for the Prometheus). I send the low pass to an 800 watt pro amplifier (NADY) and then to four 15" Acoustic Elegance (formerly Stryke Audio) subwoofers that are in an infinite baffle. Keep in mind this is an infinite baffle, not an open baffle.
Let me know if you need more information.