external crossover recommendations

What would you recommend to pull off the low pass signal and send it to a subwoofer, prior to the preamp?

I use a Velodyne SMS -1 (between pre & power amp) with great success. The unit has room analysis and PEq along with a very flexible low pass x-over and a basic high pass x-over (fixed @80hz, 12db/octave). If you need a different high pass set-up for your mains, you can afford a good one because the SMS is available on-line for $600.

Good Luck

One more thought:

Marchand is a popular choice for external x-overs. They're not crazy expensive (starting <$1K) and reportedly sound good - though I've never hear one myself.

The Rives sub-PARC. It was designed just for that and has continuous phase adjustment and parametric EQ for the bass section. It also has an amplifier for the subs.

Rives sub-PARC