I use 2 REL subs I bought used at different times, a Q150e 150 watt front firing 10" single driver, and a Q108e MKII 100 watt single 8" down firing…both from the same era, both in perfect shape costing around 200 bucks each (Ebay). I join them at the amp by sticking the appropriate wires (Canare star quad) into AQ spades and attach them all to the binding posts. 100,000 ohm ratings on each sub make them transparent to the amp’s output using REL’s "high level" Speakon input. My main speakers are relatively efficient and since changing to a 12 watt per side SE amp there is less amp signal going to the RELs now at any level (simply because the mains need less power to get the mojo), and the frequency has been set to 40 hz or so as the main speakers are good to that point (trusty test CD showed that). I personally don’t need any digital nanny DSP stuff in my listening room as the sub placement and sub level make great integration with the mains simple and effective…plus I don’t want any extra gizmos in the signal chain. Interestingly (to me anyway), with this new setup at my normal listening levels, the subs don’t seem to be working as hard (based on feeling how hot they get) and the whole rig is amazing sounding with everything I listen to, including loud classical. So, be like me, do what I do, and everything will be fine.