I just replaced the PCM56P chips with the K grade versions for a solid improvement.
$13.56 each + shipping here:
http://components.arrow.com/part/detail/84608S9189437N7713
No soldering required, just pop and plug!
Recent thread on this here:
http://forum.audiogon.com/cgi-bin/fr.pl?ddgtl&1270885459&openfrom&1&4#1
This is also discussed in the huge Havana thread on Head-Fi along with a host of other, more advanced, mods, the most common being upgrading output caps.