Hi gonk,
Here's the info I've received so far from the helpful folks at Snell:
The LCR & HCC 500 Frequency Range: 80 Hz to 20 kHz Crossover Point: 2.75 kHz.
The SUR 500 Frequency Range: 125 Hz to 20 k Hz Crossover Point: 300 Hz, 2.7 kHz.
The SUB 550 does not have an active crossover The Frequency Range: 20 Hz to 80 Hz.

I should be receiving a fax with more information. Perhaps I can scan and forward this to you, too.
Cheers