Definitely go with a sealed subbetter damping and hopefully a lower Q. Don't overlook kits. Sealed sub kits are available for well under $1k, very easy to assemble, and will give you higher performance for your dollar.
Powered subs have line level inputs. But to get the integration between the sub and mains correct you need to have the crossover set correctly. If your preamp has a subwoofer crossover and sub-out you can do it from there, but unless you're using an AV pre this probably isn't the case. So, you'll need to use the crossover in the sub (or a separate external x/o). Pre-out goes to the sub, crossover sends the mid/hi to the amp from the sub's high out. Crossover frequency should be set at about one octave above the -3 dB point of the mains. You need to have that much overlap for a linear response.
Be very careful in your selection of a sub. Too many are pseudo-subs that are barely flat to 30 Hz. To be useful, a sub needs to do 25 Hz (-3dB or better). This point can be debated, but the low A on a piano goes down to 27.5 Hz, the low C is 32.7 Hz, so flat to 30 seems reasonable.
Adding a sub will give other benefits besides low end extension. It takes a lot of stress off your mains (and amps) helping to reduce distortion and improve the lower midrange.
Powered subs have line level inputs. But to get the integration between the sub and mains correct you need to have the crossover set correctly. If your preamp has a subwoofer crossover and sub-out you can do it from there, but unless you're using an AV pre this probably isn't the case. So, you'll need to use the crossover in the sub (or a separate external x/o). Pre-out goes to the sub, crossover sends the mid/hi to the amp from the sub's high out. Crossover frequency should be set at about one octave above the -3 dB point of the mains. You need to have that much overlap for a linear response.
Be very careful in your selection of a sub. Too many are pseudo-subs that are barely flat to 30 Hz. To be useful, a sub needs to do 25 Hz (-3dB or better). This point can be debated, but the low A on a piano goes down to 27.5 Hz, the low C is 32.7 Hz, so flat to 30 seems reasonable.
Adding a sub will give other benefits besides low end extension. It takes a lot of stress off your mains (and amps) helping to reduce distortion and improve the lower midrange.