I would like to nuance this. I and another listener on this forum compared the R7 and R8HE side by side and we were unanimous. The R8HE has a very spatial sound, here the R8HE wins. We found the R7 more beautiful and richer in the mid and high section. More body, more real.
Technically speaking, the R7 is different from the R8HE with other chipsets and also different power supply for at least the digital main board. The R8HE has standard voltage regulators where the R7 has discrete class A power supplies. The analog section looks different but is discrete with the R8HE.
If the R8HE sounds much better than the R7 then I suspect that the mains voltage is to blame. Then compare the R8HE with the R7HE we have done and you will hear the benefits of both an R7 + R8HE. The R7HE remains the ultimate dac for us! The R8HE and the R7 and the R7HE all have a micro-usb upgrade port.