I'm forced to use a 100 Hz crossover with QS4s, and I use a 60 or 80 with the M50s (I don't remember which). I would use a 120 Hz cross with the Qs, but it would just be far too localizeable. I can already pick out where the sub is... With 8s, I'd probably use the 80 Hz cross.
I agree with you on the forward placement thing being a bit difficult. I haven't watched anything originally mastered in DD or DTS yet (I know, I know) but I've been going through the first season of Bab5 on DVD which was remastered from Surround to DD 5.1, and nothing's flown through my head yet, or over my shoulder, or any of that. A 3rd QS8 could fix that for you.
