Welcome
I have the 939. While I upgraded the preamp to decode the new audio codecs on BD movies, I still use the amp sections on them for the l/r and surround speakers. It has been a very faithful receiver over the years.
While the Yamaha is rated for less power, not by much, I would not expect it to be audibly different from the Onkyo within their design limits.
Was there anything wrong with the 939?