My understanding is as follows …
Tidal HiFi content is playable using Plex/Plexamp
Content exclusive to Tidal HiFi Plus (Hi-Res > 44/16 ?, MQA, Dolby Atomos, etc.) is NOT playable using Plex/Plexamp
Regardless of subscription tier, NO Tidal content can be downloaded for offline listening using Plex/Plexamp
Also note, some weirdness to be aware of …
There are some places within Plex Media Server where out-of-date terminology is used to describe the Tidal subscription tiers (if you have a Tidal HiFi subscription, it will be listed as Tidal Premium in Plex Media Server … if you have a Tidal HiFi Plus subscription, it will be listed as Tidal HiFi within Plex Media Server … you can see how this might cause confusion, especially when you’re already paying for HiFi, and Plex Media Server presents you with an option to upgrade to Tidal HiFi for twice the cost … Plex reps, please fix this, hopefully it’s as simple as changing a couple words)
There is also a known (and long-standing) issue with how Plexamp displays the codec and bitrate of Tidal HiFi content … for example, FLAC 44/16 Tidal content that has been “added” to your Plex Library will be labeled by Plexamp as AAC 320 (or sometimes as just FLAC with no numbers after it) … as far as I can tell, I’m still getting FLAC 44/16 playback as expected, so perhaps just an odd labeling issue … and if I’m understanding correctly from prior forum threads, this behavior might depend on how exactly your Tidal account is linked with Plex (external vs directly purchased thru Plex, linked vs transferred to vs initiated thru Plex, other?) … Nothing really broken here, but like the subscription tier item described above, it can make the Plex/Tidal experience confusing, and I hope the Plex team can at some point prioritize some fixes
All that being said, I enjoy using Tidal HiFi from within Plexamp (and of course I like getting a discount on my Tidal subscription) … hope the above info helps to avoid some confusion