Why did this fix my problem?

I have a deHavilland mercury preamp and Thor TPA 60 EL34 based monoblocks. My amps input impedance was 100k ohms and the Mercury is a cathode follower which I understand needs a minimum 10k ohm load. I was having a problem with the amps going into wildly fluctuating bias and would have to shut them down. Upon restart the bias would hold for a short period and then the whole process would start over. After 6 months of sending the amps to Thor who found them in perfect working condition I sent to a repair company that specializes in vintage and tube equipment. After a week on the bench the amps never acted up. I put the tech in contact with deHavilland and they decided to lower the input impedance of the amps to 47k ohms. It has been six months and I not only have never had one problem, the sound of these 2 components is just glorious. Anyone know why this worked?