I wonder if the Cambridge amp simply has a lower input impedance than some of the other amps you've been using? Basically this would mean that a slightly noisy output on one of the DACs ends up being a problem for the Cambridge units but not for the other ones you have...
Don't get me wrong; J4cbo makes a nice product, but it is priced to be affordable and with that comes the understanding that you might get a tiny bit of variation from one unit to the next. I think in this case the Cambridge amps were picking up the slightly elevated noise from the scan-speed clock in the DAC, whereas a cheaper scan amp might have well ignored it.
As far as troubleshooting goes, if you think about it - the fact that the interference changed only when you changed scan speed suggests very strongly that the DAC was the source.
Given how low the prices have fallen on used QM-2000 boards lately, you might want to consider picking one up as a "reference DAC" to be used when tuning scanners or troubleshooting projectors. Even though I own several different controllers, when I tune a set of scanners I *always* use the QM-2000 board's output.
Not trying to be a huge Pango fan boy here; just pointing out that many commercial controllers have various issues related to noise or sample jitter, either of which can mask a more fundamental problem with the projector itself. But I've never had any reason to question the QM's output...
Adam