I think that Herman is correct, if you consider shielding as part of the power supply. Most design decisions involve compromise; I expect that is true on this issue as well. Certainly more cost to go to a 2 box solution. Those $ could be used to provide a bigger, better filtered and shielded supply in a one box unit. There are excellent examples of each, for sure.
Preamp design approach - Power Supply location
Looking to hear from some design types or those who know something on this area. I have had both type of preamps and cannot myself say which is better but am very curious as to the perceived advantages of using external vs. internal power supplies (or vice versa).