preamp-how important is separate enclosure for pwr supply?

Distance between each section is more critical than physical shielding. Distance and shielding is best. 

Just to be clear I was stating my experience with solid state designs... I can't comment on what type of isolation would work for tube amps and I guess if the power supply doesn't fit in the amp box it's a bit of a no brainer.
@emergingsoul - I'm not sure what you mean by 'current issues'. If you mean the ready availability of electrical current to the amp then it's better to have the PSU as close to the load as possible i.e. in the box.

It’s important if it’s not implemented correctly in a single box design..Shielding and vibration isolation are very crucial in a preamp..
Many high end units have a separate PS chassis.. preferred, but not always practical.

If you really want to see an improvement, go with a batter powered preamp - wow!