The BC27pi is a BC27 with a power supply that would shame some power amplifiers. The difference in performance is definitely not subtle. The BC27pi has all the features of the BC27:
*Blue Circle output capacitors
*high-low gain adjustment
*HexFred rectifiers
*Stereo/mono switch
*Compact audio signal path for low noise
*All hand wired - no printed circuit boards are used in the BC27 or BC27pi.
*Low output impedance for driving long cables
*Phono overload: high gain...>9mv low gain....>60mv

PLUS:
*Over 200,000uF of capacitance in the power supply.
*Custom designed 75VA transformer in a seperate chassis.
*5 ft Blue Circle DC cord.