Are your existing amps so good that you don’t want to replace them? If so, you may want to look for a device which passes through HDMI and breaks out 6 channels of audio. Rather than the two-box approach you have above.
Something like this: https://www.amazon.co.uk/Extractor-Digital-Adjustment-Surround-Converter/dp/B0777PH1NL/ref=sr_1_15_sspa?s=electronics&ie=UTF8&qid=1536171685&sr=1-15-spons&keywords=hdmi+sound+card&psc=1
But you will have to spend more to get one that supports 4k @ 60fps.