From my audition of the apt x hd it is definitely a step up from the previous generation of Bluetooth quality. It still doesn’t compare with Ethernet or good Wi Fi. It may be better than Air Play, but I’ve never used the latter with anything better than Apple TV, so I shouldn’t even go there.
So Ish, if I now understand your question, you are asking if Streaming compressed Spotify files via Bluetooth sounds different from an iPhone vs an iPad. As it happens, I use Spotify when I travel, with B&W Bluetooth headphones, and alternate between my iPad mini and iPhone 8 as a source. I can’t say that I have noticed any significant difference between them.