I think Airplay is limited to 16bit/48khz/1440kbps CD quality, although Spotify streams at a maximum of 320kbps anyway, so there shouldn't be a significant loss of quality.
i'm guessing you've already done this, but just make sure that you've set the sound quality on Spotify to 'extreme' (320kbps) in Settings.
Personally I'm quite happy with the results. I find that the actual sound quality of the master of the particular album I'm listening to has the greatest influence on sound quality. Having said that I use Spotify for browsing new music, learning about new artists and having playlists or radio running in the background. For critical 'proper' listening I still happily purchase the CD.
I've done back to back tests on some of my most treasured music, and while I would admit that the difference is smaller than I expected (Spotify & Airplay vs CD), the Spotify source can sound a bit 'coarse' and less well-resolved compared with CD (although I should mention that I use the CD's DAC with analogue RCA input into the Pioneer receiver rather than a digital connection, so the difference could simply be put down to the contrast of two different DACs).
Have you tried the direct connection with the supplied lead? I purchased a lightning adaptor to plug in my newish iPad, but couldn't get it to play sound. Some track information made its way onto the receiver, but that's all, so I'm stuck with Airplay anyway. I did swap the lightning connector for a headphone socket to RCA phono lead, but haven't tried that yet. I can't imagine that the ipad 's internal DAC would be any better than the Pioneer's.
Something else you could try is to use the Extended Stereo setting on the receiver, then on the AV control app, set the balance to about a third of the way towards the front speakers. For my set-up this sounds fantastic.
if you come up with anything that works well, please do let me know. I've been thinking as the direct connection doesn't work on the receiver, I could always get an ipad dock once somebody brings one out that's designed for the lightning connection. Or I could save my pennies & upgrade to the Pioneer LX77!!...