[pjnad]

Check the fuses in the amp

Found the Krell manual: there are no fuses in the amp, and it is a circuit breaker on the back panel that would only engage during catastrophic failure.
The amp also has two protection modes, input/output.Based on the info supplied it seems as if the input protection mode might be engaged, that is if the amp stays on but does not output a signal it indicates a problem with the input signal.
If the output protection was engaged the amp would turn itself off...it also engages if the amp itself fails, but since it turns on and stays on, this is probably not the case.
So, I hope that I have narrowed it down to the Modwright.

[JG2]

With the SWL unplugged, check all the molex connections in the power supply side of the unit to be sure they are properly seated. If one of these is loose you will not get the rectifier to work properly.

[pjnad]

Problem solved!!!
I had written to Dan at Modwright last night and received this reply today:
Hi Paul, check the connection of the two purple wires that terminate in a white connector, that plug into the power supply board in the 9.0.  These supply power to the rectifier tube filament.  If loose or not making a good connection, it will cause exactly what you are describing.  Please let me know.
 
So, I checked, nothing obvious but I removed the connector and replaced it and voila...
thanks to all for your input...
Paul

[pjnad]

With the SWL unplugged, check all the molex connections in the power supply side of the unit to be sure they are properly seated. If one of these is loose you will not get the rectifier to work properly.

JG2: looks like you were right, I saw Dan's message just before yours...of course I would have to write to you because I was not familiar with the term "molex" 
Thanks!

[JG2]

Glad it was a simple molex connection in the PS section!

[pjnad]

Glad it was a simple molex connection in the PS section!

Me too!! I was not even sure that it was the pre...so I was still in the mode of attempting to isolate the issue, and then was mentally preparing to have to send a piece out for repair 

[modwright]

We covered this off-line. It was a matter of the rectifier filament connector being disconnected.  Problem solved.

Feel free to contact me directly when any such problems occur.

Thanks,

Dan

[modwright]

Resolved off-line.  Turned out to be a disconnected filament supply to the rectifier.   Connected put back onto PS board and all is well.

Thanks,

Dan

[pjnad]

Kudos again to Dan! I wasn't sure that he'd be available on the weekend but he answered my email that I sent on Saturday night, and was spot on with the diagnosis and fix...
I've had nothing but positive experiences with Dan and Modwright, which reinforces my decision to have chosen the preamp, when there are so many other companies and options out there.
Paul

[bakufu]

my sentiments exactly.  i've been working with dan on an issue over the last week or so, and he couldn't be more helpful.  and i love these amps (kwa 100se).