Hi, first I’d like to know what slopes on the Bryston you’re using. The frequency choices of 70/hipass and 80/lopass should make for a rather wampy response overall, but I understand the limited selection choices with your equipment. A new standalone passive crossover will not necessarily help you any if you currently have to ’guess’ at what the frequency selections should be. It would likely be much better in this regard for you if you had a unit with much more flexibility of control in order for You to experiment enough to discover what settings will work best.
(I’m assuming that you have bypassed the active crossover in the Fathoms in order to use them with the Bryston).
Give serious consideration to holding off buying a Marchand (or other crossover, whether passive, active or hybrid), or sending the Bryston in for repair, until the true nature of the audible problem becomes verifiably clear for you.
Audible "clicking" through the woofers might be digital distortion, correct me if I’m wrong here...caused by audible dropouts in volume...not usually considered harmful unless perhaps at very loud levels. That might be a digital input level issue somewhere, if that applies to your situ. If it is actually your woofers bottoming out, then this is indeed dangerous, of course. If you haven’t already, of course, double and triple check your Bryston to see why low frequencies are apparently being passed...change slopes to see if it has any audible effect or none at all. If it has no effect, then I ’might’ begin to suspect a faulty Bryston. Change other Bryston parameters too and see if the problem remains constant.
Check back in with your findings.