I already have preordered. I'm thinking the tube output design goes a something like this:
Out of the DAC > Headphone Amp
> Direct Button (Enabled) > Tube > RCA Outputs
> Direct Button (Disabled) > Volume Pot > Tube > RCA Outputs
If think the difference you're experiencing may be the volume pot coloring the music. Keep in mind, this is just my logic guessing, nothing based on facts :)
Wait what, did you pre-order via mail? I don't see any given link on his site yet.