Subscribers should be able to change video quality of non-partnered streamer
Subscribers of non-partnered-affiliate channel should have option to lower the video quality below source.
Also this is in Twitch's benefit as a marketing option to sell subscriptions, And to use less bandwidth for subscribed lurkers.
For example: If newer streamer has (at least) 1080p as source and I've subscribed to their channel then I might want the option to lurk at 480p or even 160p to show support & listen to audio, while downloading new game/updating windows/etc.
I like this idea and think it's absolutely brilliant. As an affiliate, I've had a few community members who haven't been able to watch on days that the Quality-Option-Gods haven't smiled on me due to low quality internet, and having the option for a Twitch Prime sub (for example) to allow for personal quality options in those cases would alleviate those issues. The idea that a subscriber should 'always' have access to the quality options of a channel they're subscribed to is a great one.
Making quality options available for subscribers, across the board, might even be something to consider, however would impact basic viewership, and shouldn't be considered lightly.