I think you need to force the receiver into one of the 7 channel matrix modes -- Dolby Digital EX or DTS ES. Haven't played with a Yamaha 1500 but I imagine if you just keep pushing surround buttons you'll eventually get sound out the back speakers. Here's a brief blurb from one of Alan's articles :
"Recently, Dolby Labs added an extra channel to its Dolby Digital 5.1 channel scheme and called it Dolby Digital EX. Not to be outdone, DTS came up with its own version, DTS-ES. If your receiver has these modes, it will decode the small but growing number of movie soundtracks encoded with a back surround channel reproduced by one or two speakers behind the listener. If one, it’s called a "6.1"-channel system; if two, a "7.1"-channel setup. The idea of this extra channel(s) is to provide a more immersive listening experience and greater realism with more convincing surround effects. While the extra channel is not a truly discrete digital channel, it nonetheless will deliver a more seamless surround effect and better coverage, especially in larger rooms. If your receiver is equipped with Dolby Digital EX and DTS ES decoding, and you’ve added the extra one or two speakers at the back, when you play a DVD movie that’s been mixed for EX or ES, the receiver will automatically decode the extra information. You don’t have to do anything (this is good). And even if you play a standard 5.1-channel movie, your receiver will automatically extrapolate information for the extra one or two rear speakers. There is a further wrinkle with DTS, an ES mode called "ES Discrete". Some newer discs so-encoded deliver six entirely discrete channels plus a subwoofer channel. (With Dolby Digital EX, the extra channel is "matrixed"; DTS also has an ES-Matrix mode.)"
http://www.axiomaudio.com/archives/surroundsound.html
Apologies if you have already tried these modes, wasn't sure from your post. Good luck !!