The December 2008 issue of Affordable Audio has a well written review of the Ultimate PS MKIV by John Hoffman
http://www.affordableaudio.org/He has been reviewing variations on the SqueezeBox mods for about a year.
The end of the article has some nice summations:
"The Bolder Cable Co. Ultimate IV with Nirvana regulator resides on the frontier of power supply design for the
Squeezebox...The latest version of the Ultimate power supply offers improvements to the performance of the Squeezebox that cannot be
achieved in any other manner. The Ultimate IV increases the low level detail of the SB3, and also improves the lower
registers of this player...For audio enthusiasts looking to obtain the finest digital playback available from their Squeezebox, this power supply is a logical choice." 
