It looks like you are set on the Aperion speakers! I don't want to sound like I am trying to talk you out of buying those, but there is a couple things you should know about power handling with respect to speakers before you make that purchase. Check
this article out, it explains it pretty well. What it partly means is the loudness difference between the Onkyo 708 and 808 will be negligable. Even if you had them driven by a two hundred watt amplifier (the max the speakers are rated for), it still wouldn't make that big of a difference from the 708, at least with respect to SPL. Here is a
speaker peak SPL calculator to understand where your speakers will get you. For 110 watts (the onkyo TX-sr708), the Aperion 6B's will get to 100 db at 6 feet. That's pretty loud, if it were a continuous sound, but not really loud for peaks. For what you want to accomplish, the peaks are what is relevant. To give you some perspective, I often crank my electronic music louder then that, but I do not go clubbing because most clubs are painfully loud to me.
Here is something else to consider, your desired Onkyo's were measured for 110 watts x two channels, not five channels. When you add more speakers, that power really drops, so eventually you will not even get 100 db. Also keep in mind that you will want to keep to Aperion for the front stage speakers to have them timbre matched, and their center channel speaker is a bit pricey.
Like I said, I don't want to discourage you from getting those if they are what you want, I just want you to know what you are getting. The reason why their sensitivity is so low,
as it was explained to me by a Aperion rep, is for the low frequency extension, but since you are getting subwoofers to handle those low frequencies, that factor becomes irrelevant. Honestly, I think they would be good speakers, but if it were me though, for dubstep I would look for speakers with a sensitivity rating better than 90 db. But, of course, everyone's tastes are different, what might be too loud for some isn't loud enough for others. I think you should try them out, just remember to test them to see if they can get to the loudness you want before the 30 day return policy is up.
