The difference is there, its more evident on more sensitive IEM's. To be honest I wouldn't be able to tell the difference between a fuze and a sflo2 but definitely tell a difference between the sflo2 and an ipod classic.
Again, the Fuze is an excellent PMP. Comparing an sflo2 to it would be foolish on my part cause I cannot tell the difference between the two in terms of SQ. The Sflo2 definitely has more power to drive cans like the M50 with ease which the Fuze does with a little more effort. But with balanced armature iems, as long its the same song, same bitrate, same volume, I doubt anyone can tell the difference between the two. With cans it would be easier to tell them apart.