I just popped in a pair of Mullard CV4024s into the PI and reverb driver slots of my new (to me) Fargen Blackbird 40 combo and the improvement in tone over the JJs that were in there is quite noticeable. The amp sounded pretty damn good before but the Mullards seemed to open things up in the mids and gave better detail in the highs, the low end is nice and taught too. Going back to the JJ ECC 81s made the amp sound congested and a bit boomy by comparison. I have seen others say how much they liked these tubes and I just wanted to say that I do too. And thanks to KCA for getting them to me so quickly (2 days!). Oh, and the Fargen Blackbird 40 is a fantastic amp. Shane.