Low speaker volume for B Side Speakers...

I hooked up a buddies Sony Home theatre recently in which I have a separate set of environmental speakers wired to the front B side wired with a volume switch on the patio. The home theatre works well with the Bose Acousta Mass system and powered sub. However, when I switch to the B speakers - I get the sub still on but minimal volume is directed to the outside speakers. If I put A+B on reduced sound is everywhere. It seems the amp is set up to have B speakers only used as an additional front speakers in surround operation. I would have expected the B side would be able to be used as 2 channel speakers for audio only or in concert with the A side with equal power to all speakers.

Is this a setting on the amp that will fix this? Or is it more practical to buy a separate amp for outside and use the Home theatre amp as a pre amp and wire the second amp through the 2nd Source audio out?