Hi, I m using the SR5015 from last 7+ months with my KEF Q150s and I'm satisfied with the music in stereo mode and also Pure direct mode. I haven't extensively used a stereo amp, so I can't for sure how much gain would be there with a dedicated stereo. I would suggest you pick the 5015 and use it for a while and if you feel there's something lacking, pickup a integrated amp a bit later.
I'm planning on upgrading the speakers to Q550s, or something else.. let's see how that goes.
On a side note, how's your experience with Q500?