you an idiot? 128 aac sounds the same as a cd, the compression takes away noise the ear can't hear, so essentially your going to all that trouble of buying a cd for extra sound quality that your dog can hear...
Actually, there's quite a big difference between a CD and 128kbps AAC.
That massive difference in file size means information is lost. EVEN if AAC encoding ONLY applied a digital high pass filter at 20hz (it does way more!), the mere fact that it's processing the signal another time means things will sound worse for it. MP3, AAC, and all other forms of lossy compression can do some really nasty things to your audio.
Here's a comparison for you :
http://www6.tomshardware.com/consumer/20020712/2u4u-05.html
Also, there has been significant research into the effects of equipment that can handle signals greater than 20hz to 20khz (which, by the way is usually much more than most adults can 'hear'). What happens when you change or discard frequencies outside this range is interesting... it can affect other frequencies at multiples (ie. 2x, 4x, 8x, or half the frequency etc). So you 'feel' the change. It affects the sense of quality, as well as imaging. When you know what to listen for, it can become VERY obvious. This one of the main reasons is why the music industry now uses frequencies of up to 192khz and 24bit to record. Once you've heard a recording like this on good speakers, listening to a CD is actually not all that amazing. With good dithering (POW-r, Apogee UV22 etc), CDs can be pretty close to the original though.
Of interest, it's usually imaging that's lost--the subtle differences between left and right channels that give a sense of depth to the recording.
So, in some situations, it'll be hard to hear the difference, but there's plenty of us that WILL be able to hear. I think you might need some better speakers.