What is the best way to add subs in my system?


Hello fellow audio people!
I have a great sounding system, but want to bring it up a notch. My ATC SCM 19's are the best speakers I've ever owned; extremely transparent with phenomenal dynamics. They also need quite a stout amp. ATC recommends at least 75 watts. I was driving them with a big Parasound A21 and an NAD pre-amp. All in all a nice combo. Then I saw a Luxman L550 ax here on A'gon and had to have it. With 20 watts/channel I'm only using it as a pre-amp for the Parasound. What I'd like to do is run the Lux direct, and add a pair of small sealed subs to take full of advantage of those sublime 20 watts. What is my best bet for a line-level crossover? JL audio has a new one out but it's extremely expensive. With signal purity my top priority, how can I add a pair of subs to my system without breaking the bank? More info on my system in virtual systems,
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I've promoted a parallel connection from the speaker taps for a long time. I also am a big fan of dual subs. I use inexpensive but well-designed (the Phil Jones model) Pioneer 8in subs with Fostex full-range drivers in a medium size 17x35 room.
Fabulous information, all! I just glanced over the thoughtful responses now while on a quick work break but I'll read and look them over more closely later today!
Thanks again....
Audio Kinesis Swarms a good value http://www.audiokinesis.com/the-swarm-subwoofer-system-1.html
+1 Rythmik subs, with a DEQX. Two subs are best. The DEQX is the only way I have successfully crossover subs. This also lightens the load on you primary amps.