Conclusions
All in all, I was impressed by the quality of MP3's vs CD. Unlike the preconceived notions I had when I set out to do these tests, the 192Kbps MP3 was indeed "near CD quality." The degradation was indeed subtle, but noticeable. It is my belief, however, that on the HiFi setup the 192Kbps MP3 is more fatiguing to the ears during extended listening. Surprisingly, 256Kbps and 320Kbps MP3 were virtually indistinguishable by my ears from the CD, except for a light boost in the upper midrange and treble.
So what does this all mean? My feeling is that 192Kbps MP3 is more than adequate for listening with a computer or in a car. The distortion and lousy frequency response of the systems themselves (as well as the high ambient noise in a car) make the 192Kbps MP3 sound so similar to the original CD that the advantage of the smaller storage requirements of the MP3 far outweigh the sonic benefits of the CD. It's nice to have 5-6 audio CD's worth of music on a single MP3 CD - almost like having a CD changer in your hands. On the other hand, for critical listening on a HiFi, I feel that 256Kbps is the best bitrate to use, because it sounds significantly better than 192Kbps. Since I couldn't hear any improvement at all when going to up to 320Kbps, I don't recommend going this high for the simple reason that the file sizes are significantly larger.
MP3's ripped with BladeENC sound far better than I expected, and I believe that most listeners (except neurotic audiophiles those with extremely expensive equipment who claim that $800/meter speaker cables sound significantly better than my cheap but high quality 12 gauge stranded copper cables) would find that the difference in audio quality between the original CD's and 256Kbps MP3 is insignificant.
Update
Putting my controlled testing aside, for practical purposes, maybe it doesn't really matter if lower bitrate MP3's sound inferior to CD's. For casual listening (e.g. on PA systems, trashy built-in laptop speakers, or in noisy environments, such as in a convertible with the top down) 128Kbps is probably more than adequate. I have done some further informal tests with non-critical listeners, and they really don't notice (nor do they care about) the difference in audio quality between CD's and 128Kbps MP3's.
Sometimes, it is beneficial to put aside scientific/engineering mumbo jumbo, and instead analyze the cost/benefit ratio. Given a fixed amount of storage, one can store considerably twice as much music at 128Kbps than at 256Kbps. Before deciding what bitrate to use, analyze the target audience and listening environment, and then make a educated decision as to what bitrate is appropriate. 128Kbps has been chosen by many people because it's of sufficient quality for the vast majority of listeners. If you like to listen on a portable device with small storage, such as a 256MB flash-based MP3 player, you can fit more songs on the device if you use a lower bitrate. Anything lower than 128Kbps is unacceptable, IMHO. Personally, I will stick with 192Kbps for all but my most treasured pieces, which I will encode @ 256Kbps, but each to their own.
