12-08-13: Zd542Interesting. I'm thinking that the reason for that statement, and for some of the problems that have been reported, might be the effect that Ralph (Atmasphere) described in his two posts starting here, which relates to interactions that may occur between amplifiers having very extended low frequency bandwidth and preamps having less than ideal stability in their power supplies. I note that the specified 3db bandwidth of the Ref 110 extends to a very low 0.6 Hz. I note also that Ralph mentions ARC specifically in his second post, although that experience occurred many moons ago.
It does say on the first page of ARC's tube amp owners manuals that: ARC tube amps must be used with ARC preamps or damage will occur. I guess they put it in bold print for a reason.
Another possibility, of course, might be that ARC is anticipating that some users will not follow the proper turn-on and turn-off sequences (amp on last and off first), which might result in the amp having to process large transients that may be generated by some preamps at turn-on or turn-off.
Regards,
-- Al