The problem moved to the other channel when you switched to tubes.  

You can now slowly narrow it down to the actual weak tube by putting them back the way they were one at a time and testing the amp after each one.

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Replace the weak tube or tubes and you should be back in business.  

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NOTE:  If the culprit turns out to be an output tube, it is always best to purchase output tubes in matched pairs.  Especially in a push pull amplifier, as loosely matched pairs will cause hum on whatever channel they are installed in. 
 

 

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