Yeah you are right. Maybe I'll just go 6.1, although there aren't very many 6.1 out there either. What is good for the additional speaker for a 6.1 setup?
Multiple people posting at the same time makes things interesting

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I am a little confused now that I have read the other responses and then looked at your signature. Your signature lists your HD7's as "rears"; is that correct, or are they really used today as your side speakers?
In any case, I would not bother with a 6.1 system; that poor speaker would mainly be a dust collector. Even a 7.1 system that is not designed correctly (all speakers good, and in the correct location) is not all that great. I know that from personal experience

, my rear speakers are 1) no good, and 2) in the wrong location. However, do to a variety of reasons, I cannot make any changes to speakers 6 & 7 (BTW-not included in my signature).
Back to your basic question - - try the big speakers and see if you like the results. Nothing much to lose except time and the pain of running the cabling. At one time I had some former front speakers (12 inchers) used as sides and they worked OK. The main problem is that they set to low for proper side speaker placement (way to heavy for above ear level

), but it was the best I could do at the time.