Shawn and I have C4S'd the production prototype - woo hoo! We put C4S active loads on the 12AU7 drivers last night - and easy upgrade - and got a really nice improvement in dynamic punch. Today we took on the more challenging job of putting together a test C4S for the 6080 cathode. That required our biggest Avid heat sinks on the MJE 340s and a little adjusting of the 6080 cathode current down to allow for the additional draw of the bias LEDs on the active loads. After making sure it all worked exactly as planned and test firing it on my HD600s we put PBs HD800s on. Wow. I'm going to have to go back and compare it with my big reference headphone amp again. So far our loads are holding up and the power trans is running at the right level of warmth. We've got the 600s back on right now and we're just letting the amp run. Keeping our fingers crossed that we have achieved enough cooling for it to run happily for several hours.