I was really disapointed by quality of sound when my FreeRunner was playing. Complete lack of basses made it impossible to use as a portable player. There is an article on Wiki, describing what is wrong and how to fix it:
I was thinkig how to push 100uF cap into the metal can. My solution is to take two 47uF capacitors (those suggested by Andy Green on wiki) and connect them in parallel to get 94uF.
Figure 1: Bass-fixed Freerunner
There is even enough space for heat shrinking tube so the capacitors are well isolated from the metal can and the rest of components.
Figure 2: Bass-fixed Freerunner - in detail
As seen on Figure 2, original 1uF capacitors are unsoldered and wires from two 47uF caps hidden in heatshrinks are soldered to their pads. But it is not necessary, wires from caps could be soldered to the original cap's terminals. My initial idea was to solder the 47uF caps to the place where 1uF were, but there is no space for it.
Part number of the capacitor is 12066D476MAT2A. It is 47uF, 6.3V ceramic cap in 1206 package. I got it from Farnell: http://uk.farnell.com/avx/12066d476mat2a/capacitor-47-uf-6-3v-1206-x5r/dp/1658886
Hope this helps!