Re: Noob requesting feedback from smart people.
First, the easy one: it's not a surprise me that the "compressed music enhancer" (or whatever Marantz calls it) is actually bad. Most of these circuits do EQ that I find makes any music, compressed or not, sound worse.
The other things you're asking about are the subjects of great religious wars in the audiophile world. At the risk of being branded a heretic by one side or the other, I will attempt to walk the fine line here.
For a long time, people on the "objective" side have been saying that, more or less, all amps sound the same. Most of them don't mean this literally, more like "all similarly-spec'd solid state amps of the same class sound essentially the same." I won't get into my stance on this but you can google this and see millions of electrons have died on either side of this cause.
More recently, many folks in the objective camp have been saying that "all DACs sound the same" and there's less qualification offered on that than in the amp space. By and large, the claim goes, the technology for DACs has been "solved" and there's no difference between any two modern DACs. I think that position is perhaps a bit extreme, so let me just say this: the difference between any two reasonably-competent DACs is pretty small. I believe I can hear small differences between crappy cheap DACs and "good" DACs, but I haven't blind-tested myself so I'm not 100% sure. IMO, there may be some gain by adding a good outboard DAC, but I don't think the gain would be substantial.
Compressed music vs. full-resolution music is another controversy. (BTW, music from iTunes is not "Mp3 AAC," those two descriptors are mutually exclusive--a file is either an mp3 or an AAC file. iTunes music is AAC.) Anyway, you can definitely find folks out there who argue that people can't tell the difference here, either. This one I do have direct blind-testing experience with, and the data say that indeed, it's really hard to tell. However, sometimes I can if I listen really carefully. I think there are three things at play: (1) recording quality, (2) equipment, and (3) experience.
On (1), there's a lot of music in my library that I can blind test on pretty much anything and I can't tell the difference better than chance at all. It depends on what the music is in terms of content, how well it's mastered, and whether the key features are masked by other sounds or not. Bad 80s drum machines, for example, sound just as fake at 256 as they do at full resolution.
On (2), I think this is highly dependent on the quality of the reproduction. I'm positive I absolutely cannot tell the difference in a noisy environment (e.g., in the car) or with equipment that lacks resolving power. I definitely own headphones where I cannot tell at all, no matter what the music is. I'm sure that's true for several of the less-capable speakers in the house. However, with some material I can usually do better than chance on a blind test with my Sierras or with better headphones. I'm sure the RAAL towers are more than good enough here.
On (3), you have to know what to listen for. Compression tends to create certain artifacts, and if you know what to listen for, it's easier to find it. Back in the days of 128-bit files, I couldn't stand to listen to a lot of music compressed because the cymbal smearing was so bad; once I knew what to listen for I couldn't not hear it. Modern compression algorithms are much better and at AAC 256 or mp3 320, it's often really tough to tell.
TLDR: whatever difference there is, it's not huge.
Things like mastering and speaker placement make a bigger difference than these small effects. For example, difference between the original CD release of Dark Side of the Moon and the MFSL version is massively larger than the difference between uncompressed and 256 AAC, or between any two decent DACs. I recently bought new speaker stands that raised my Sierras to a more correct height and that made way more difference than changing DACs ever did.
Luna Duo V2 LR, Titan Horizon V2, and Rythmik L22 & L12 in HT, Sierra-LXs in study, S-2EXs and Duo V2 C in bedroom, S-1 NrTs in dining room, S-1s at work, HTM-200s in kitchen. Brother owns CMT-340s and dad has a pair of CBM-170s.