Stone, trust me, there has been much controversy regarding the Axiom 500 and 600 and the SVS Ultra. The HSU VTF-3 HO Turbo has now been thrown into the mix as well. And, yes, those subs are more of an even match. If you could afford any of those over the VTF-3 MKII, I would recommend going in that direction. But, if you can't, you needn't worry about the HSU VTF-3. It is an excellent product and will do the job just fine.
After a quick look at the Yamaha manual, I believe those crossover choices are "global." That means that whatever crossover point you chose is where ALL the speakers are crossed. I would start with the 80 Hz setting. Feel free to experiment with other settings if you wish, but that setting will work fine with all the Axiom speakers. The higher you move the setting (100, 120, etc.), the greater the risk of the bass being locatable (i.e. sounding like it's coming from the subwoofer, rather than blending with the speakers.
I think your choices are just fine. GO FOR IT!
Not an option to send the "unused power" from the rear speaker amplifiers to the mains. Think of your receiver's amplifier section as 7 distinct individual little amplifiers.
Optimally, the QS8s are placed on the side walls slightly behind the listening area and between 1 to 6 feet above your ears. The Axiom stands are meant as a compromise for those who, for any reason, absolutely cannot mount them on the walls, or suspend them from the ceiling. Using them is less than ideal, but not unsatisfactory.
