Ok the preamp stopped cutting out frequently and I did have the volume up at about 65%, I would never want it any louder...
However it did cut out at low (30%) volume once after about an hour of listening.... (so the problem seems less but not solved)
I will try to get my brother to find the manual for me...or perhaps Ill contact Arcam.
Are you using the new CD player? Is the problem continuing?
I'm really reaching here (beyond what I can consider my experience/knowledge-base), but as a long shot...
I have a Marantz receiver which has a red "Peak" light that comes on if the input signal is too strong for the pre-amp unit. The manual says I need to engage the attenuation (20dB reduction, I think) function to ensure no problems with overloading the pre-amp. So, my wild-ashed guess is that you may be doing this when you feed from the CD player. Your pre-amp may go into a protection mode when this happens. Try turning the cd volume up to 100%. If this SWAG is correct, it should shut off quickly, and turning down the cd to ~40% and cranking the pre-amp should work without problems. For this situation, the pre-amp volume setting wouldn't matter - only the cd volume setting would matter.
The other option is to "reboot" the pre-amp. Usually there is some way to reset the pre to the original default settings (which are chosen to result in the least likelihood of the pre being returned because it didn't appear to work properly).
On the screws, It sounds like the rust is peppered across the surface? I was at first thinking it might be where a bit engaged it and marred the coating.
If so, it is possible that the screws were a poorly coated. Because of the greater load on the woofer, it would be normal to use longer screws for the woofer than for the tweeter, so that would explain how one set might rust while the other didn't.
Just some more thoughts, and guaranteed to be worth every bit of what you paid for them!