[TLDMan]

Exactly what the question asks. I don't listen to the FLAC files themselves (I use iTunes as a player) but I convert them to ALAC and listen to them. I'm not blown away by the quality like people say, though, and I can't really tell the difference between ALAC and 256kbps or so (though my setup is a pair of PortaPros connected to a set of $40 speakers connected to my PC, I'm not sure if that's the problem.)
 

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You could ignore everything I said above, and just answer the question: is FLAC sonically different from ALAC at all?

 

[QRomo]

They're both lossless codecs, so the decoded audio should be identical to the original source for both. The only way for them to sound different is if the encoder or player has some sort of bug.

 

[khaos974]

It could be the mastering itself, IMHO, a well mastered album in 170+ kbps mp3 sounds better than a badly mastered edition of the same album in ALAC/FLAC/WAV.
Of course, a lossless rip of the well mastered edition would even be better.
 
To answer your question, both ALAC and FLAC are decoded into raw PCM before playback, and this raw PCM is strictly identical, so they sound the same.

 

[XXII]

They should sound identical.
 
It seems to me that you're expecting too much from ALAC/FLAC. All it does is makes sure what you're listening to is not any worse than listening to the CD -- if the CD is recorded poorly or otherwise doesn't sound good, the lossless rips won't sound any better.

 

[Ham Sandwich]

The difference between lossless and 256 kbps lossy is very subtle.  It is near the point where it would likely be audibly transparent or nearly audibly transparent.  The difference between lossless and 128 kbps lossy will be more, but still subtle.  They're the kind of subtle difference that you may have to train yourself to hear and recognize.  So I'm not surprised at all that you're not hearing a difference between 256 kbps and lossless, especially with the gear you've got.  There are people who can't hear the difference with even much better gear.

 

[kiteki]

I used the PortaPro's for quite a long time,  I think they're quite well suited for lo-fi like a cassette tape player or playing PS2 games.  They are too muddy and closed-in to discern the difference between lossy and lossless.  They are more for gettings jives from the radio or a 128kbps mp3 collection.
I adjusted to the muddiness as I usually adjust to whatever I'm using, but now if I were to venture back and use them for my portable Flac I doubt I'd hear any difference and I'd quickly press "next" or "off".

 

[EYEdROP]

In theory, they should sound the same because they are both lossless files. Like people were saying, even a 256 or 320 MP3 is quite good enough, especially for an album that sounds like crap in the first place. Lossless files make more difference if the album was well recorded.  
 
It seems like the treble range is most affected by using compression and mp3. Cymbals become washed out and the air is gone. 
 
lossy vs. lossless is about as subtle as changing sample rates in my expeirience (although I dont have a nice dac). IMO, anything over 256kbps is listenable enough if I cant find a flac version. 

 

[Mad Max]

With the same music encoded into both formats, you shouldn't hear a difference between the two.