[shch13]

Now of course Apple doesn't use FLAC, it uses ALAC (Apple Lossless), now I'm assuming based on research they are essentially the same, except for that one thing I picked up about ALAC being easier on battery life.

I've played a FLAC file ripped from a CD (via Sony MediaGo) and placed it on my Walkman Z, a rockboxed E383, and also placed an ALAC version of it on my iPod Class and Shuffle.
Using the exact same IEM on the 4 devices (a Sennheiser CX270; Yes I know its not exactly hi-fi, but i'm more mid-fi with my range of IEMs/headphones with XBA3 being my highest end).

To my ears it seems as if though the iPod's produced less screeching(didnt reach the floor ceiling) as quickly as the Walkman's did. Mind you there were still times where both encountered screeching at those higher pitches, but the iPods encountered it less than the Walkmans.
 
Bottom line:
The iPod had fewer times where it encountered the problem but when it did it was exactly at the same moment of the track as on the Walkmans.

Is it my source gear problem? Is it the lossless file?
 
The track that I played is Jpop R+B artist: Misia - Everything.
Pretty sure the youtube version prob wont produce the same sort of ceiling given the compressed audio.

 

[shch13]

I've found a work-around. I've ripped the WAV file version of it and it cuts down on the screeching, on the player and also if listened to on PC.
I've also ripped another version of it to 320 MP3 with LAME, and that also cuts down on the screeching, though not as much as the WAV.