Strage problem - power lines amps /preamp noise

So for the longest time I thought I have had bad luck with some used amps that I have been buying here... While my current amp; SF Power 1, sounds good, it is also very quiet (no hum, no noise from the transformers, powers up clean w/o any pop etc..). Several other amps, both tube and ss, would make noises; loud pop during power up, hiss and noise thru speakers etc.. I was guessing it may be power but just over the last few days of experimentation I realized that it is a bit more complicated.
So again; my current system (SF1 to Cary SLP-03) makes no noise whatsoever. Once I have a different amp connected I hear a white noise / hiss thru speakers with left channel being more pronounced.
What I have discovered is that if I disconnect my preamp (not just power off but unplugged) - the hiss is no longer present. BUT...I can also eliminate the hiss by simply plugging the preamp to another outlet, HOWEVER, after an hour or so of playing the noise would come back and then all I have to do is move the preamp to the previous outlet and the noise would disappear again...but again only for about an hour.

All outlets I tried are on the same circuit... I also tried (just for the heck of it) computer UPS unit with the same effect.

What gives? Any idea if there is something wrong with the preamp? My power lines? Combination of both?

Ether - I should have mentioned that my pre-amp is plugged directly into an APC surge protection/battery back-up device as are all the other components except for the two amps which are plugged directly into the wall with each having its own dedicated circuit. Likewise the APC is plugged directly into its own dedicated circuit. It is only the monoblock amps that require the plug converters.
Could be a lot of things. Some questions:

1)Are you using balanced connections between the preamp and all of the amps that you have tried?

2)When the noise returns after an hour, instead of moving the preamp's AC plug to a different outlet have you tried disconnecting it for a similar amount of time, and re-plugging it into the SAME outlet?

3)Does the hiss level vary with volume setting?

4)Are you following the proper turn-on and turn-off sequence, i.e., power amp on last and off first?

5)Is there anything in the house that might be turning on and off every hour or so, that could be generating RFI (radio frequency interference) or a transient on the power lines when it turns on or off?

-- Al
1. Balanced interconnects from CDP to Preamp to Amp
2. Yes - This happens if plugged back into the same outlet
3. No, hiss level stays the same regardless of volume setting, except if I turn the preamp to 0 (mute) - then it disapear as if I turned it off.
4. Yes... but this has no influece on the symptoms (I tried different sequences)
5. Can't think of anything like that...

Thanks for any and all input !!
CORRECTION: Reading my above post, I realized that my Answer #3 is misleading.
So the hiss is not heard when I lower the volume to 0 but it is back on at the same level when I just turn the volume up.... it only goes away for 1 hr or so ONLY if I completely unplug the unit and wait some time before plugging back in.
That is puzzling indeed.

All I can think of is that there is something defective in the preamp, perhaps a capacitor or a tube, that (a)is temperature sensitive, and therefore does not occur until the unit has been operated for some time; and (b)for some reason is not induced by the SF amp, but is induced by the other amps.

Just speculating, but perhaps (b) relates to differences in the manner in which the amps connect (or don't connect) circuit ground to AC safety ground. For diagnosis purposes, you might try temporarily using a cheater plug to isolate the safety ground pin on the power plug of either the preamp or one of the amps that brings out the problem. I'm not envisioning that the problem is a ground loop, since apparently no hum is present, but perhaps something related to how the grounds are handled in the amps other than the SF plays a role in triggering the problem within the preamp.

-- Al
All I can think of is that there is something defective in the preamp, perhaps a capacitor or a tube, that (a)is temperature sensitive, and therefore does not occur until the unit has been operated for some time
I have no electrical engineering knowledge at all, so take this for what it's worth, but an intermittent problem that develops after a period of operation and then goes away once a unit is shut off, only to re-appear, always makes me suspect a temperature sensitive defect.
It sounds like the Cary might have a tube, or something that gets noisier with heat. But, trying the below options might help.

On the electric part, try plugging in a portable electric heater in the same outlet, and see if the noise starts quicker due to the homes electrical parts warming quicker. This may help diagnose the outlet circuit wiring, and the possibility of a circuit breaker (in breaker panel for home) making poor contact after some warming up.

Also, try a 25 to 50 foot, maybe a 14 gauge or 12 gauge extension cord, and try plugging it into different outlets in other rooms. One may be on the other phase leg, and not have the noise in it, if it is an electrical problem.

A poor connection could occur at your electric service entrance also. A neutral in the breaker box, meter socket, or the utility wire connecting your house. The utility company might check your meter connections, and their connection supplying you, if you really think it's there. Light bulbs getting too bright and burning out would be a sign of a poor neutral connection in this area.
For those using a computer UPS (like an APC) connected to your audio system, REMOVE IT!.

Those APC units (and most others) produce square wave AC when they go on back-up, which produces a lot of high frequency garbage you do not want in your audio power supplies. They are great for PC switching supplies, but not so for audio. If you must use a battery backup, you need to find ones that produce a true low distortion AC waveform, and these are rare and very expensive.

If you want line transient protection, a product like the Furman surge protectors (these don't have battery back-up) is much better.