I had 5k's at the rear and they were very good, i changed them for 6k's and that was amazing, everything just sounds so much better, i would say get the best speakers you can at the rear, try to match the fronts if you can.

861v8+ID41, HD621, DSP6000, DSP5500HC, DSP6000, 2x DSW's, Roon, AudioQuest Niagara 5000, IsoTek Titan, Nova, Vision, Mira.