Has anyone did a blind test on iTunes AAC vs CD/Lo

Yes. You are correct 256 AAC is pretty darn good.

I'm curious to see if anyone on here, with a really high end system, can tell the difference.

Sorry for the bad grammar... Has anyone done" is what I meant

I wouldn't call my system "really high end," but I could tell the difference between AAC and Redbook. I had some extra time tonight, so I created 256 AAC versions of some lossless rips, burned them to CD, and had my wife place them in my changer so I wouldn't know which disc was in which slot. The music was Autechre and Cocteau Twins. I could tell the difference with CT almost immediately, the AAC compression sounded a little dull, Redbook had more detail and depth. Autechre was much harder to tell a difference, I had to isolate a certain spot on the first track; on the Redbook version, I could hear more of an etched sound as the synthetic effects began and ended, again there was more detail. But it was subtle. Overall the soundstage was pretty much identical between the two formats.
I burned the AAC files as audio files, I don't know if this would affect the sound, it might have, but I doubt it. I have upgraded my 128 AAC purchases from iTunes to 256, and those 256 AAC files sound very good. I am sure whoever compresses music for sale on iTunes has access to much better compression software than what I have on my laptop. I would rather have had done the test with these files from iTunes, but alas, I don't have Redbook versions of these 256 AAC tracks.

Good info. I only tested with iTunes downloads and cd. I never tried comparing a burned CD-r. Eventualli, I hope, Apple switches iTunes to apple lossless. I highly doubt it with the iCloud service coming up. I believe all iTunes material is stored on Apple's servers at full resolution. The encoding and downsampling occurs during the download process (thats what I've heard)

I'm curious to see if anyone on here, with a really high end system, can tell the difference.

You don't need a really high end system (although it helps). You simply need to know what artifacts to listen for. I can hear audio compression on a boombox or car system.

Hmm, did not know the iTunes music gets compressed on download, that's interesting. I still think a 256 AAC purchased from iTunes sounds better than one I create myself, there's some tech there that I don't have access to. Of course I haven't tested this with the same track, its just a general statement right now, my impression. I might test this in the future, but for now, I prefer to buy CDs. I like having the media, and I have shelving to fill!

I'm the same way. I prefer physical media. I have pretty sensitive ears and can spot major compression artifacts quickly. But, i'm at a loss with an iTunes downloaded track. At least with the system I have now. I wonder if the above poster actually listened to an ACTUAL iTunes 256 AAC download. If he is hearing artifacts on those files he has some golden ears. I can id pre echo, mild wobbling or distortion with lower resolution tracks as with MP3's at 320. I know the artifacts exist on the AAC's, I just do not have the ability to hear them (yet).

Great topic!
I have been burning at AAC 256, 320, and lossless. I have Martin Logans which are pretty revealing. In blind testing I can hear no differences between CD and lossless, possibly minute differences between CD and 320 AAC but it is extremely close. Slight changes start to appear at 256 and below in terms of high end and low end rolloff, loss of room size, and lack of air. I have settled on AAC 320 for my ipod and using the Pure i-20 dock, have replaced my CD player as my source.

The guys over at hydrogenaudio strongly disagree with this and state that in double blind testing with the latest encoders, AAC 128 and higher is 100% transparent.

I've only heard a 128 Mp3 and there is a big difference over CD. I've never tried a 128 AAC

AAC 128 vs. Redbook on my system... Its not even close. Redbook wins by a long shot. I'll pick the 128kbps rip every time in every blind test, I'll bet. I don't see how anyone can say 128kbps is 100% transparent. But like Shadorne says, you have to know what to listen for. I guess not everyone can (or wants to) tune into those lost details at the low and high end, that is where I hear it. I also hear it during lapses in the music where the song is reduced to a single instrument, like a guitar riff or a bass drop, or a sound effect. Low bit-rate rips kill detail in those passages.
Consider that the bit rate of a lossless AIFF runs at 1411kbps, this is 11 times the bit rate of a 128 AAC. Just like in video compression, quality is all about the bit rate. How much data can the codec push. I rip apple lossless.

Realremo, I do not think anyone disagrees about 128 AAC. 256 AAC is the resolution in question.