i have also cary 40M monoblocks.
i tested a lot of EL34 brands
The svetlana and the chinese shunguang where for me the
best new production sounding El-34 tubes.
The cary produce some huming even with matched tubes.
With the shuguangs they where complitely silent.
By the way are you happy with the sound of the 40M?
I found for my self that bass extension is not Top...
Any comment?
regards
sam