Single ended to XLR Question

I'm really stumped on this one. I have recently purchased a pair of ATC fully active speakers which require a balanced XLR input at the speaker. The dealer recommended a set of cables with single ended outs from the preamp, so running single ended to xlr. When the speakers are powered up with the preamp OFF (McIntosh c2300) the speakers have a very audible hum. When the preamp is turned on the speakers are dead quite. I used the same cables with another preamp (Aesthetix) and the speakers are dead quite when the preamp is OFF! I then tried a fully balanced pair of cables (xlr to xlr) with the McIntosh preamp and dead quiet with the preamp off. So why would the single ended cable create no noise with the Aesthetix preamp in the off position and do the opposite with the MAC. Could it be that the Mac isn't a fully balanced premp? One of those things I can live with but trying to understand.....Thankyou.
It's probably just a matter of what the output impedances of the two preamps happen to do when they are turned off.

Probably the output impedance of the Mc in the off state is high, allowing pin 2 (the non-inverted input signal line) of the ATC's xlr input connector to "float" in an uncontrolled manner, while xlr pin 3 (the inverted signal line) may be exposed to inter-chassis hum voltages caused by ground-loop effects. Since balanced input circuits respond to the difference in voltage between those pins, hum will result.

The Aesthetix probably has a lower output impedance when it is powered down, resulting in pin 2 being "pulled" to a voltage level closer to the Aesthetix's ground, which in turn would reduce the difference between the voltages on pin 2 and pin 3 of the ATC's xlr connector (pin 3 presumably being connected to ground in the cable).

One thing I would do, though, is to use a multimeter to verify that pin 3 of the cable's xlr connector is connected to pin 1 (the ground pin). That is what should be done by a cable or adapter that is used to drive an xlr INPUT with a single-ended signal. (Pin 3 should ideally not be grounded, btw, when an xlr OUTPUT is adapted to drive a single-ended input, although in practice it is frequently done, for no good reason IMO).

Also, I would make a point of comparing sonics using the balanced and the unbalanced outputs of the Mc. If the balanced driver circuit in the Mc is of good quality, I would expect it to be preferable. I'm not sure why the dealer would have steered you to the single-ended cable.

Finally, as you may realize it is good practice in a system that has passive speakers to turn the power amp on last and off first, relative to the preamp and other components, to prevent turn-on or turn-off transients that may be generated by the preamp or upstream components from being applied to the speakers. Similarly, it would be good practice with your active speakers to turn them on last and off first, which would eliminate the hum issue.

-- Al
Have you tried plugging the speaker power cords into the same receptacle as the rest of the equipment ?
Thank you for the feedback. It really isn't an issue as long as the preamp is turned on first and turned off last. Just was puzzled as to why I would have 2 different results from 2 different preamps.