Marantz SR7011 for Stereo

I have a pair of KEF R900 connected to a Marantz SR7011. Although an AV Receiver, it does a decent job in terms of Stereo performance. My main sources are Spotify and Youtube streamed via Airplay. Im also planning for a CD and Vinyl player.
The issue is I feel the Stereo performance could be better considering the Speakers handling capacity. I have been contemplating on the following two options to better the Stereo performance :
1. Connect a Power Amp to the Marantz. Been looking at Parasound 2250, Rotel 1590 etc. any suggestions would be helpful. If connecting the Power Amp doesn’t improve much, then adding a Pre Amp (with HT bypass) to the above.
3. Connecting an Integrated Stereo Amplifier (with HT bypass) to the Marantz. The amp to be of suitable enough to drive the KEF and also include airplay. 

Appreciate receiving suggestions on the above please. As for power amplifier I have been unable to decide on the right model and hence any advice will help. 

I was in the same position, as I have a single room for HT and 2 channel.

I started with a separate amp too, made an improvement, with my AVR as a pre in 2 channel. Still not satisfied, finally got a separate  preamp for all 2 channel sources with an HT pass-thru when watching TV,  and now 2 channel is substantially better.

If you want good 2 channel, don’t use your AVR. Either do as above, or perhaps an integrated as you mentioned, with an HT pass-thru if your room is dual purpose as mine is.