All I know is that if the original recordings were done in analog and they were transferred to 24 Bit (AIFF), I've compared plenty to 16 bit Redbook, AIFF and ACC files and 24 bit kicked the living crap out of everything else. It wasn't even a contest. But what do I know, I'm just listening to it and even on a decent stereo, nothing super fancy. It's OBVIOUSLY better. Better bass definition, better ability to hear subtleties of the original recording, much better clarity, etc. etc. Everything you want. Obviously, there are a lot of recordings that were done originally in 16 Bit digital and there isn't much they can do to improve it that.
I did read that several top mastering engineers thought Mastered For iTunes is totally acceptable and there really isn't much of a difference in that and lossless. They newer software is better than the original process.
I've done some listening tests between the newer 16 bit AAC (Mastered for iTunes) and Redbook 16 bit ripped to AIFF and I couldn't hear any noticeable difference. But when you get into 24 bit vs 16 bit, they are typically very noticeable. I talked to and read interviews of top mastering engineers that do mastering of HD Tracks and they have mentioned they have updated their converters to what they used to use and they much better s/n ratio, dynamic range, etc. so the converters are getting far better than they used to use. I know there were a few bad recordings that got onto HD Tracks in the beginning, but I think they pulled those. But everything I've downloaded is far better than 16 Bit where I ripped a CD to my mac to AIFF. But I'm talking in generalities. Yeah, I'm sure if the mastering studio had a crappy 24 bit converter compared to a high end 16 Bit converter, but that generally does't happen. A lot of earlier 16 Bit CDs really sucked. I mean they were horrific and sometimes they remastered them using better converters and they sounded better, but I have yet to hear a 24 bit recording that isn't better than 16 bit from HD Tracks. I'm sure there are some that are out there, but I haven't heard it yet. I have about 20 recordings from them so far and a growing collection from B&W's site. The biggest problem is digital recordings originally done in 16 bit. They can't do much with those.
I just do the listening test on my system, if there is a difference and it's noticeable then it's noticeable, if it's not then it's not. For every single 24 bit download I've done so far from HD Tracks, i heard a VERY noticeable difference than a AIFF from CD. And the different was night and day. Once I download the 24 bit version and it's that much better, then I just delete the old or just keep for listening to on my iPad or iPhone since they only do 16 bit. But for my computer based stereo? 24 bit is the way to go. go listen to Santana Abraxis. much better than 16 bit lossless.
And again, you're ascribing a lot of importance to 24-bit, when you have no basis of comparison other than CDs that were likely mastered under completely different conditions. A remastered CD can produce the exact same results that you describe. Lower frequencies depend the least on high resolution, so any differences you observe with bass performance can almost assuredly originate with differences in EQ or other processing used in the mastering.
The true basis of comparison is when you can find a CD and high res track that were transferred at the same time or under very similar conditions. I've done those comparisons and it's not a "night and day" difference. Yes, I have observed differences when doing these comparisons, but they pale by comparison with the mastering and other post production steps.