kef r900

I have a pair of KEF R900 that is connected to a Marantz SR 7011. The Stereo performance is not up to the mark being an AV Receiver. I intend to add either an Integrated Stereo Amp or Seperates (Power and Pre) with HT bypass to the preouts of the Marantz. Have been looking at the below options...

My Marantz SR7007 is paired with KEF R900 floorstanders and KEF R200C centre. Jamo E8 sub just for some additional low end. I'm not running any surround speakers. At lower volumes, this pairing works just fine and sounds amazing. However if I crank up the volume (to around 70-75 on the...