adding a sub

What you will find is there are many ways to connect a sub (or a pair of subs). My perfered way is different than Kal's.

I prefer to use subs with speaker level connections - you connect the subs to the same binding posts on the back of your amps (or even the binding posts on the back of the speakers). Just double up the connections.

I prefer subs with variable crossovers that give you crossover frequency options of 30 Hz at a minimum. You'll probably get advice to cross your subs over around 80 Hz. IMHO, that's the best way to hate the sound of subs. For a starting point, multiply that -3 dB down point on your speakers by 0.70 to establish your base crossover frequency of your subs. As always, YMMV.

Also, I prefer subs that give you 0-degrees, 90-degrees and 180-degrees of phase control. IMHO, 2 subs are better than one.

Parts-Express:Dayton Reference drivers and the 240W plate amp.Allows high and low level input and adjustable crossover,level and phase.The 1K ohm/high level input will not effect the impedance that your amp will see.

My sub sounds much better using the speaker-level connections. the input impedance of the amp for th sub has such high impedance that it will not affect you ramp or full-range speakers at all.

Go with Kal's recommendation. It''s more general and practical than the others. Also, 80 Hz is likely to your best bet for crossover. There's rationale behind that choice, not mysticism. If you require symmetry for psychological reasons, go with two, but a single larger sub may be more satisfactory.

db

There's rationale behind that choice, not mysticism.

You are right, there is rationale...it's called THX specifications.

If I may quote:

It is the product of much development, and, when used in concert with THX speakers (or others which exhibit the correct roll-off), represents the best overall compromise of minimizing localization, extending dynamic range, and as it turns out, minimizing LFE truncation. When Dolby Digital was coming to the consumer marketplace, THX looked at an inordinate number of modern 5.1 soundtracks and guess what they found in the LFE channel: not much at all in the region of 80 Hz - 120 Hz, making their original choice of 80Hz rather fortuitous. Dolby Digital's LFE channel has a digital brick wall at 120 Hz, not a roll-off, so content creators almost always roll-off their stuff, usually somewhere around 80 Hz. Therefore, chucking the top band of the LFE is no big deal but the argument here is that a standard SSP crossover set much lower than 80Hz or so may actually be costing you bass content.

http://www.micasamm.com/pages/info/florian_0103.htm

However, if you are getting great 2-channel sound with a sub crossed over at 80 Hz, the more power to you.