So I just got my 1974 SVT in the mail a few days ago. WOW! What a beast of an amp. After a thorough cleaning and checking of parts, I decided to power it up. It sounded great but was making some occasional whooshing sounds and tapping on the preamp chassis made a pinging noise. Today I dug deeper into it and ran the amp without the preamp through the power amp in and everything sounds as it should. In the preamp, I found that the 6C4 driver tube is extremely microphonic and most likely needs to be replaced. What would you guys recommend getting for a replacement? Ive heard that most all of these tubes have microphonic qualities but are some brands are a bit better than others? Also should I buy a few of them NOS and then see which one works best? Thanks!