Supernait 2 protection circuit

Have you tried swapping the amps over so that the SN2 is driving the bass section and see if the problem persists.

If you go the 252 route, a 300 (if able to drive these ok) would be a better way to go than bi-amping with a pair of 250's. 

Have you tried single wiring using just the NAP250?  In general, with a NAP250 or above you needn't worry too much about the nature of the speaker; provided that you have at least 3.5m of NAC A5, they'll happily drive a 2Ω load.  With some speakers, bi-amping can cause weird interaction effects.

Thanks James N and Huge, I'll try your suggestions to see if it makes a difference.  I bought the 250 because the Supernait cut out before and I thought having two power amps would help by sharing the load.  I think just the 250 on its own will be OK as its' the Supernait cutting out and muting the signal to the 250 as well.

Out of interest, why do you recommend the 300, is it just better suited to the 252 or is it a completely different design?  I did think of buying a 300 but as I mentioned above, I thought bi-amping but be a better for my speakers.

Sorry, I meant to ask if I need change any settings/sockets on the Supernait if I'm just using it as a pre-amp?

Thanks again.

No, just disconnect the speaker cables from the SN.  You need it's own power supply connected and to keep the link plug as you need it to power the preamp itself.

Thanks again HUGE, your suggestion worked a treat!