1. Should I use 3tis or QS8 at the side?
QS8's for sure.
2. Are my existing 3tis sufficient in the rear or will I need something beefier? I could, for instance, forego the side speakers for now and instead buy two pairs of M60s (one pair for the front, one for the rear)
M3ti will be great for the rear. M60's would be a stupid amount of overkill.
3. I take it M80s are really not necessary, and M60s are sufficient in this long room?
Depends on how loud you like it. I'm guessing "regular" ceilings? If so, our rooms are probably comparable in cubic feet, and I honestly never find my M60's wanting.
