I just received my ZP3, but it didn't ship with a rectifier tube.  I don't have any 5Y3GT tubes currently, so I tried using a 5U4.  When I powered on the ZP3, without any input connected as instructed, I get a noticeable hum out of the speakers.  

I always float the grounds, so the first thing I tried was grounding various components, but that only added more noise.  I also changed every tube including using a 5AR4, and nothing helps.

The manual says to only use the 5Y3GT rectifier, yet the pictures on the site show a 5U4.  Could not using a 5Y3GT be causing the hum?

Also, one more question: Can I use an OD3 instead of the OA3 as I do on the Torii?  I like the tighter bass of the OD3, but wasn't sure how sensitive the ZP3 is to changing tubes.  

The hum might be because I turned the volume up on my amp to about half way.  And it's more of a hiss noise, not a true 60Hz hum or anything.  

Last night I had some problems that my 0D3 and 0C3 regulator tubes both were flashing when the ZP3 was fully powered on.  My initial inspection of the circuit showed two 220 kOhm resistors in parallel between the output of the regulator and the ground.  My quick calculations came out at < 1 mA of current, which is too low for the regulator.  If the values of the resistors were 22 kOhm instead, the current would be where it needs to be.  

I'm either paranoid and making things up, or something could be wrong.  I think it's probably time to call Steve...