YW!
Generally speaking, if something is labeled Hi-Res, its master should be taken as close to or equal to studio quality as possible. That's my take on it anyway.
I've compared enough of my Hi-Res purchases with previous Hi-Res, but more so CDQ and def mp3 copies, and as I'd said, the overwhelming majority of the time, the Hi-Res version sounds the best. Sometimes you'll hear it on every song, sometimes maybe only on certain ones. It's never gonna be the same every single time.
With regards to how Apple handles its Hi-Res masters, I remember reading up on it a few years back, and
this document by Apple pretty much answers everything you seem to be looking for, so check it out.
Sure, you can try that. Those Zep Hi-Res remasters were all done very well (produced by Zep's lead guitarist himself, Jimmy Page, who was also very involved in the original studio recordings), so any of them will do. But might as well start with the debut! The Fleetwood Mac ('75-'82 era), Eagles, Tom Petty, The Who, and The Rolling Stones ones are fantastic too. Other more recent Hi-Res greats that I have include Lenny Kravitz's "Are You Gonna Go My Way" album, the Pearl Jam ones, U2's "Achtung Baby" (which is w/o a doubt one of my all-time favorite albums), etc. Or any of the ones I'd mentioned a few posts ago too. I'd prob start with Zep though, just b/c they're my fav band (along with The Rolling Stones). lol On that debut, aside from "Babe", "Your Time Is Gonna Come" is another one I consistently use to sample new headphones, including the comparison I'm doing now. The drums that come in after the organ intro are by themselves fantastic!
If any of those other bands are ones you like, lemme know and I can check and see what songs I've used and let you know which ones they are as well.
Yeah, I like convenience too. I mean, I use a SEMI-wireless setup for my premium listening (since the connection b/t my phone and xDSD is BT), though it outdid my old LG V20's "Quad DAC" FULLY-wired setup. And also when I workout I use my BT Samsung Galaxy Buds+, but IMO, when you're working out, you're simply NOT gonna be listening as closely or as seriously as when you're chillin'. And if you are, you're not working out hard enough. LOL But the AKG-tuned Buds+ sound great anyway esp w/ my custom EQ. Not "headphone" great, but not far off at all.
Some ppl don't hear a difference from streaming. And that's fine. I do though, so for me, I keep it to a minimum, like for previewing albums I may wanna purchase. If you don't though after a few tries, then just stick with streaming. No sense in forcing it. But if you do, then purchase only the songs or albums that really move you. Leave the rest for streaming. lol