Preamp/Amp impedance missmatch?

Thanks for the emperical testing! My preamp was indeed an older model (at least 3 years old). I'll be talking to you soon about getting a newer one.

I tried to duplicate this with my own Kora pre amp by removing the power cord during play, shutting of the power supply etc. and was not able to duplicate the problem. I am a dealer and spent some time discussing the problem on the phone with Eadese. His unit was an older unit with different power supply connections and some other differences. This may be the problem as the older ones also had a switch on the back of the power supply unit designed to eliminate ground noise if present when used with certain amplifiers. The new ones have a new grounding scheme which eliminated the need for this switch.

Thanks for the response... I was thinking I was the only person every to have this problem. The Kora I owned had a mute delay that worked great on power up. The problem always occurred when power was removed.

Eadese I also had problems blowing power amp fuses, due to AC power interruption @the preamp. My situation was a bit different (a weighty upgrade AC cord would occassionally become disconnected) and the resulting turn-off transient on the audio output lines was enough to DC-clip the power amp resulting in blown rail fuses. Although your situation occurs somewhat differently, the result is the same. One question that would be helpful to know: do the PA fuses blow upon preamp power-down or power-up? You can find that out via some simple failure-simulation testing.
If the failures occur upon preamp power-up, then one way around this is to delay the amplifier's turn-on until the preamp output initially settles. I have heard of AC conditioner that has internal on-delays to the PA power receptacle (Panamax perhaps? - or others?) but then you're dealing with possible sonic degradation if the conditioner constrains dynamics (many reportedly do) so you'd have to chose carefully. Or you could build an accessory box containing a heavy-current relay & a short-delay timer, quality heavy gauge AC cord, quality internal wiring, & perhaps an FIM AC outlet, or other of reasonably good quality. The box would again delay energizing the amp until the preamp settles.
Another possibility which you've already explored would be a different preamp that doesn't behave as the Kora does. Perhaps the newer model works better? Or you might consult with the manufacturer to determine if they have a fix for this bug? One brand with which I have personal experience is Accuphase. This preamp contains an internal muting relay circuit that, upon power-up, doesn't connect the audio output sections to the panel jacks until after an internal time-delay executes, thus the preamp has settled before connection is made to the PA. Upon power-down the muting engages immediately, again isolating the output jacks from internal circuitry. I can turn this preamp on or off anytime without fear of any power amp consequences.