I think I agree with Yomanze.
When the information is not there in the recording/mastering, you'll end up with a flat soundstage, no matter what.
When I upgraded my streamer this spring, the soundstage on several recordings was increased trmendously, on others, nothing had changed.
I also heard that a slightly better streamer with a pair of Klangedangs, which are very small speakers. Soundstage wasn't great. We added a small sub, and the soundstage improved significantly, beyond belief actually. So adding bass performance was very important, but I have no idea if the same would have happened if the speakers themselves would have been worse.
Oh, and this may or may not be relevant: room modes in both my room and the additional had been taken care of.
Mileages may vary