That Pyle should work ok..but I have had mixed (pardon the pun) luck with Pyle's products..some work well and over the long haul, others have failed and let me down. If that seller is a legit Pyle dealer and offers full warranty it would be worth a try..otherwise steer clear.
The transformer I recommended is cheaper and as long as the Yamaha receiver isn't blown out, it will work fine and "future proof" any later replacements.
The Muzak (TM) box my wife gave me was mono. Maybe they do stereo but not at her place. It also had 70v and regular 8 ohm outputs.
The 70v was bad but the 8 ohm still worked. My neighbor is using it now. I coupled the stereo line level input from an MP3 player through a ground loop filter (pair of transformers) to a "2 for one" RCA cable and it seems fine. I was concerned about tying to outputs together directly too.
Greg, generaly speaking, when you are at "line level" (Before the amplifier inputs) is is ok to tie outputs together. It is AFTER the amplifier (speaker outputs) that tying together is a very bad idea.