I built an amp for a steel/guitar player with 2 KT88's running over 600v on the plates (625?) about 10 years ago, never had any issues. 425-0-425 power transformer with solid state rectifier. It was a detuned Trainwreck/Bassman circuit, 1650R Hammond output transformer, 5k primary impedance. You can tell your amp guy if you'd like. I even have a few power transformers yet, I might do another KT88 build but higher gain. The tubes were JJ's. I'm not really a fan of JJ's but these held up well, only one needed to be replaced because it was rattling.