How sub-satellite system should work - crossovers

I'd second Arni's suggestion about using a configuration that leaves your existing speaker connections to the Celestions as is and running a second set of speaker cables from your amp to the high-level inputs of a powered sub. The second set of cables, the ones to the sub, don't need to be of the same gauge as your main cables since the powered sub is drawing very little current over them, meaning you don't have the expense of two sets of high-quality cables as you would if you go first to the sub and then on to your mains.

If you have only one set of speaker terminals on your amp but have five-way binding posts you can use spade connectors for to cables to your Celestions and banana plugs for the cables going to the sub.

I've used that setup with a pair of ProAc monitors and an HSU VTF-2 sub and found it provided easier sonic integration than using a high-level connection from pre-outs on the amp.

pinnacle has a very nice little sub called the subsonic that will work well as well as only take up about a 9" footprint; ie, pair of 6.5" drivers

their babyboomer is good, too, and only a little larger

my first choice would be a REL Strata II or III, though

There are many ways of doing this, all depending on your other components and preferences. If you don't have a receiver/processor where you can pre-set the crossover freq. to the Celestion D100 as "small," then getting a sub with internal high pass filter (most of the active subs do have it, and it is generally set at about 80Hz), would work fine (you would connect the sub to the speaker output on your amp/receiver, and then the speakers to the speaker outputs on the sub). However, if you generally do not play loud, and you are basically looking to augment the low end of the D100, then you would maintain better sound quality by leaving the speakers connected as they are now, and then connect the sub separately, either via additional pre-out, or via the speaker connectors on your amp/receiver. If you do more music than movies, don't play loud, and the transparency of your system is important to you, then I would recommend this way of connection with a sub with smaller drivers (10" or less). Check out the smaller subs from HSU and Pinnacle, these are excellent values.