The tube with ZP on it is causing the crackling. I put it in with one of my old tubes and it was crackling straight away. The other one is quiet.
I got a replacement tube last week and all is well. The amp sounds good. I built a crossover with an attenuator so I can knock the edginess off the horns. I ordered a miniDSP 2x4 for biamping, so I am looking forward to that. I think I'm going to use my Onkyo receiver for the time being for the bass, I've been using it as my preamp. I have a Magnavox I could use for the tweeters, I'll have to scrape the dust off and rig an on off switch. I finished up my ST-70 and i could use that for the bass, as well. I still need to source another Mark III. These things are crazy money unless they are all rusted. Fair condition is $400-$500. I have another set of matched PSVane KT88's coming from China via Ebay, for when I get another one. They were only $65 and the guy has a ton of feedback. The miniDSP looks to be a bit of a learning curve.