Giving up on Itunes! Alternatives? I want to run my hardrive with a dedicated tablet.

you are asking a lot of questions (good, but real answers are complex and lengthy).
You got great input on Audirvahna and Roon. I use Roon.
This is a sacred cow, but IMO cables are vastly over rated and over priced - difference or not, the money can be better spent elsewhere. yes you need a quality cable, with good shielding and the proper impedance. And good dialectic material.  pretty much full stop. I will now be crucified. I'm in good company.
DAC are HUGE.  As noted, its a whole topic but DACs perform timing, decoding, its a preamp, its a filter, its a [power supply... and all matter. IMNSHO opinion a lot.  Full disclosure i design this stuff for money (but its a side gig mostly for fun).
OTOH the law of diminishing returns is alive and well.  You can get a DAC that will kick the crap out of your tablet/laptop/PC's internal DAC for $99 - and its Made in USA (Schiit modi 3). But step up to $500+ and you get substantially better. Can you spend $25K?  yes. Should you? Well, is  Porsche GT3RS a nice plaything?  yes, it is. Is it the best cost/comfort/benefit trade off for an occasional blast through the twisties, or an occasional Autocross? Nope.  But god bless you if the $250k is not enough to stop the purchase. get two and drop one off at my place.
Do some reading and listening.  Go to Youtube and watch some stuff by Hans Beekhuysen. He's really good.  Kooky like most hgih end audiophiles, myself included, but good. And he does not subscribe to magic, which endears him to me.
Enjoy.  Happy to answer specific questions, but i avoid the endless opinion and flame wars that seem to occur regularly.
G

someone said: "iTunes/Apple Music isn’t noted for its audio quality."
Incorrect!   Itunes on a mac mini is one of the best playback platforms, assuming you are playing HD or FLAC (ALAC) files.  Couple it with BitPerfect ($10) and you have a terrific (sounding) solution.  Unfortunately, its cataloging is pretty useless especialyl for complec Classical.
Even Paul, the owner of PS audio, admits that rather than use his own megabuck transport or streamer at home, he often uses a mac mini with Bitperfect.
I did this (on a laptop, not a mini) for about a year until i wanted to throw itunes out the window - but not for sound reasons.
I now use Roon.

Larry - i think maybe you are confusing itunes ( an app) with music formats? if you are clear on the difference, please make it clear - not everyone will take the correct learning from simple statements. You can play HD in iTunes/bitperfect - the decoding occurs in the DAC.
For those who don;t know how to play HD on iTunes - either use bitperfect (strongly suggested by me and all of $10) or rad this:
https://www.pooraudiophile.com/2016/02/how-to-play-high-resolution-audio-with-apple-itunes.html

As to MQA MQA- no. It does not support the first unfold to the best of my knowledge. But I’ll also debate the value of MQA on most recordings. And in fact I’m listing to Tidal based MQA right now as I type, and have compared it to my own FLACs of the same masterings. note that FLAC and ALAC are the same except one is big-endian and the other little-endian.

note I am not getting the 2nd unfold because none of my DACs contain the licensed MQA chip. You didn’t mention what DAC you are using and whether yours does. Note also that the DACs that do support MQA in the hardware are not among my fav’s at their price points -- so this is a trade off too.
Now back to the original question - you can get very good performance up to CD quality out of iTunes with bitperfect (necessary for the best performance). As to why you would want to? Well, i don’t. Its cataloging is terrible for classical, and it has real issues with multiple versions of ANY album. And its interface is simplistic.

But fundamental sound is not its weakness.
Now this brings up a good point about files - you certainly want to avoid MP3 for the best quality - even at 320 kbps they are quite lossy. Sometimes euphonically so tho....
As to HD in general (24/96+) - this is a can of worms. Its probably necessary for the studio where a) levels may be mis-set and b) post processing will be done. In the final playback - its at best a 2nd order issue. Great masterings of 16/44 can be superb. Ive noted many such recordings in he past (and many are 50 years old by the way). And crap recordings (listening to velvet underground in MQA at the moment) cannot be saved by HD. they just suck.
I think that roon and audirvahna are good suggestions.

G

update - i see that Roon now supports both unfolds, or so it seems, so I may be GTG simply by updating settings for each DAC device/endpoint.  I'll report back.

ok, can;t edit my earlier post. Roon does NOT support unfold #2, despite some exciting, but misleading, wording.  Too much to hope for.  Gotta buy Bob's chip.