TL;DR: Are AV receivers / AV amps that upmix from 5.1 --> 7.1 still a 'thing'?
I have googled
av amplifier with 5.1 upmix
av amplifier with dolby IIx
Most of the stuff that comes up seems out-of-date, and I can't find any currently available AV amps that advertise Dolby Pro Logic IIx.