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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The Vandersteen crossovers are not line level but speaker level. Seems like a seamless path to me. I have had Vandy 3A's with 1 then 2 subs as well as my current Quatro's. Quite a bit cheaper as well
Speaker level is the way to go. I use a pair of Gallo TR1D subs approx. 600.00 ea in my winter system. I run my main speakers full range and tap the signal from the speakers to the subs and use the built in crossover. If you want to limit the power to your main speakers, then wire to the Gallo subs and use the built in crossover line outs to your main speakers.

The Gallo subs are sealed and extremely compact. In my room they put the punch into kick drum and go down solid to 25hz.
There are a pair of Rythmik F12's new on Audiogon this morning, $1000 the pair. The F12 is a great, great sub.
Just for fun have you tried running the Lux direct to your ATC's as is? You might be surprised at how well those 20 watts do running full range. Plus it costs you nothing to give it a try.
I use Rythmik subs, so I'll second that recommendation. SVS also makes a good value subwoofer at about $1K/pair (and a pair is better than a single sub IMO).

As to the best way to cross the subs, I'm going to go off script here and recommend a pre-pro with Audyssey. I've tried a lot of different approaches using pre-amps from ARC and Joule (among others) as well as some pretty expensive high-end x-overs, including Marchand. My Onkyo pre-pro is IMO far better sounding than any of the high-end set-ups. You may be stripped of your audiophile club card if you follow this advice, but I think it's the best way to go.

I have been running the JL CR-1 crossover for almost a year now with a pair of Focal 1007BE monitors and a JL e112 sub. To my ears it is very transparent and works exceptionally well. Lightens the load on the speakers which is what you might, and I say might need to run the Luxman with 20 watts. Yes it is expensive but is very good. I would first do as one of the other posts suggested and just try the Lux as is. It may just surprise you!
Dont get to hung up on the price of the subs . The size of your room will dictate a lot . I picked up a used Paradigm Seismic 10 sub recently . New retail a dozen years ago was around $1200 . Picked it up for $250 . Bought it for the hell of it , seemed crazy low price . Inserted in my main system with King Sound electrostatic speakers and has completely filled in the 30 hertz range seamlessly . How loud you turn the volume up is the biggest mistake made usually . Keep it really low . I would stay in the pro arena for a sub . Less fancy looking (pay less)
lots of great information here:

if you want to use an active XO, jl audio, bryston and marchand are the most common. if you're just looking for a line level high pass filter, Vandersteen makes a decent one (HP-5) but be aware of the slope - I believe it's only first order.

Another consideration is proper time alignment of the sub and mains. I ignored this for a long time, simply high passing my mains with the afore mentioned line level unit from Vandersteen, then a custom built set using V-Caps. Worked well enough, but I found a significant improvement in integration when I properly time aligned the mains and sub. Things really snapped into place perfectly - there is absolutely no differentiation between the sub and mains, just one perfectly cohesive soundfield.
Oranfoster---If a shallow-sloped (6dB/octave) x/o works for you, you can make (or have made) the most transparent one possible with a simple r/c filter. Just a single resister and capacitor per speaker, installed on the inputs jacks of your loudspeaker power amp. The value of the r & c is determined by 1- the input impedance of your amp, and 2- the x/o frequency you desire. The formula for computing the r & c values is all over the net.
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
That would be Andrew Jones designed subs.
+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.
Well, this is where I ended up: a Bryston 10B active sub and two JL Audio Dominion d108's. I'll have them installed next week!