Yeah, I have a working DIY Pass F5 up at Marc's place. I've yet to hear it, but I need to get up there and bring it home to try at some point. I do expect that to be a step up from the Opera, especially now that I have the Luminous Audio passive preamps.
I bet the 300B is lovely, but that's in the $5000.00 range, yes?
Taking my current K1K rig over to a friends house in about 45 minutes. He is a non audiophile, but he has listened to some of my gear for a few minutes here and there. Never the K1K yet though.