AVR 435 question

Hi all, fantastic website.
I have a question that may seem simple for you guys:
I bought an HK AVR 435 (7.1 channel) and hooked it up to the HKTS8 6.1 system. I used the EzSet/EQ to set it up but the initial menu only gives me the option of setting to 5.1 or 7.1.

When using 7.1 it marks error as it is not detecting one speaker so I can not continue with the set up. I set it up for 5.1. but there is no sound coming from the rear-central speaker whatsoever. Is there a way I can use the 6.1 system with the AVR? Any input will be appreciate it.


Thanks in advance

C