preamp-how important is separate enclosure for pwr supply?

It really depends on the amount of effort the designer has gone to mitigate the emi/rfi inside the chassis of the amplifier. It will also be dependent on the components used in the amplifier. In my amplifiers I use toroidal transformers with electrostatic screening between the windings and a grain oriented steel band (sometimes called a belly band) around the circumference. This works for me with no measurable interference above the noise floor and certainly nothing audible. By far the biggest impact is where the transformer is situated within the chassis and how it is oriented as the magnetic fields are quite focussed. That's all part of the design and development process.
The advantage to me is that I know exactly where the transformer will be in relation to the circuitry (rather than in another box) and I can be sure that the amplifier will perform as designed.
Doing all of that is quite expensive and most customers won't appreciate the cost and effort that's gone into it so I can see why it may be preferable for some manufacturers to put the power supply in a different box and charge for it.

Pragmasi,
thanks. Beyond noise I thought the designers may take on current issues.   
Although noise floor measureability seems to be the clear way to judge and if there is no measurable interference then a separate box don’t seem to make sense and preferable to reduce taking up valuable space.

The power supply is always critical!


We have five power transformers in the power supply of our MP-1. As a preamp in a single box, the box would have been heavy and enormous. Even with the use of toroids we've seen radiated magnetic fields which could be picked up by the phono section, so it seemed prudent to put all that in a separate chassis. We don't use steel since steel is ferro-magnetic; we found that it sounds better when we use something that's non-magnetic. Of course that means there's no shielding at audio frequencies, but since we use fully differential balanced circuitry that's really not been a problem.

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