So I'm pretty sure the tubes in my Invective are going out. At band volume the sound fluctuates in and out, but not at lower bedroom volume. Sounds like a tube issue, yes? The Invective manual says it will run KT88's so I think after hearing the hype for these tubes for years I'm going to try some. What difference should I expect in a 5150 type amp between 88"s and 6l6's? More punch? A difference in the midst? What brand? I've used JJ's the last few years, but I wouldn't mind trying another brand. And what should I put in the V1(clean tone tube slot yes?) to add a little more high end sparkle? Thanks all!