Subs with active crossover

I have some monitors I like a lot (Meadowlark Vireos), but they are not designed for the deep bass in the electronic, ambient and film soundtracks that I listen to. I'd like to hear about Agon members' experience with subs that use active crossovers to filter out frequencies below, say, 60 hz. I know REL is the favored brand, but am curious to hear about other (maybe cheaper) brands. Are there other subs out there that can do this with no audible impact on sound quality from the mains? And will this always require connecting my speaker wires to the sub?
"with no audible impact on sound quality from the mains"
That is half of the million dollar question. The other half is getting a sub to blend with the mains at the crossover frequency and getting good placement to avoid the boom!

Certainly there are two way active crossovers that would go between your pre and amp. This is probably the "cleanest" way to go. Merchand, IRD audio, Paradigm make some of these. Price used $150 to $600.

Hypex sub amps have variable high and low pass filters (24db/Octave) so you could look for those subs or use one in a DIY. The Rava is the only sub I know that uses one.

The other option is to let you mains run full range, find the natural roll off with a spl meter and a test disc and bring the sub in just below their natural roll off.

Hope this helps,
I have been looking into the same with speakers and subs especially external cross-overs and a designer friend of mine offers a few points on what to look for on the subject on cross-overs, he makes a good point on integration. He likes the Marchand equipment for the pirce. When discussing the controls he wanted me to look for ones that have phase and delay controls for subs or woofer towers to time-align with the main speakers.
Trying to spend less than a REL costs is going to get you nothing but disappointment, IMHO. Good subwoofers are inherently expensive, and cheap subwoofers are invariably worse than nothing at all. That's just the way it is. And by the time you put together an active xover, a good amp, and a good passive sub, you've easily exceeded the cost of the comparable REL.

You should be spending as much on the subwoofer(s) as your speakers cost, or you will end up disappointed because the sub won't blend musically. The money is well spent, though, because if you're careful you will end up with a system that sounds better than if you had bought full-range speakers that cost twice as much.

I haven't heard many other subs that blend as well as the RELs, and none that do as well in the low-cost (~$1k) range. If you want to avoid hooking a REL up to your amp (this is done with a cable that REL supplies, not your speaker wires), consider the Vandersteen 2W series, which work pretty well and are fairly cheap. The other good option in the low-price range is the ACI Titan.