HELP 7.1 sound in a 6.1 system

[Octavio]

Hi, I have a problem listening to 7.1 tracks with my Onkyo receiver 605.
I have a 6.1 HT system, and I am not sure I could place another speaker in my studio (it's a small room).

However I thought that I'd hear 7.1 tracks downmixed to 6.1 but I cannot. There is no sound coming out from my surround back speaker. The only way to hearing my SB speaker is by selecting on the Onkyo receiver 2 SB speakers, not only 1.
Ok when I select 2 SB speakers I got sound from that speaker but I don't think it's downmixed, most likely I am listening just one side of the SB speakers (left side I guess).

Is there some way to hear 7.1 audio tracks in a 6.1 system (downmixed of course) instead of getting only 5.1 audio or have I to buy 2 SB speakers?

Thank you in advance for your kind replys. and excuse my poor english.