Hi,
The answer can also be found in the BP-26 manual located here;
http://www.bryston.com/PDF/Manuals/300018[BP26].pdf
Page 4, bottom grey rectangle.
Quote;
"
Although all of the above specifications can be met regardless of whether the BP26 preamplifier
is positioned immediately above or below the MPS2 power supply, optimum performance
will be achieved when the power supply is positioned above the preamplifier
"