By that I mean 6L6's in amps designed for EL34's and vice versa.
First evidence of this was my old Laney GH100TI, which ripped with 6L6's. Then came the 6534+. Much as I love the 6505/5150 sound, damn it sounds so much better with EL34's. Next up is my JCM900, typical Marshall preamp with a 6L6 power section.
Latest evidence? Well I was lucky enough to get an iPad for Xmas, so I've naturally bought Bias and I'm pretty much swapping power tubes on my custom models. The Recto was nice, but it's sooo much better with the EL34 model on it. Actually really, really makes me want to try EL34's in a real Dual Recto (or just get a chance to fuck about on a Road King).
I just wish Bias had a KT88 tube option.
My "Mini Rectifier" custom model in BIAS sounds awesome with EL84's. I love the app.