Tube amp crackle noise with pre-amp volume at zero.

I changed out some stock EL 34s with a brand x on my prima luna Dialogue HP amp. Within seconds of powering up the amp with the pre-amp volume all the way down, I herd a crackle like noise from the speaker.  It was somewhat intermittent and random between the two speakers.  I shut it down, checked all connections, and started again with the same results.

I returned the stock tubes and everything is normal.  What happened?  I didn't have the guts to increase the pre-amp output! Tube microphonics? (spelling?)