Well I'm going to do a big 'U' turn today: The reason I ended up at 100Hz was due to a colouration from my centre speaker which is really positioned too close to the floor. At 80Hz it gets an extra bit of bass and on louder sections it starts to sound 'muddy' so I settled on 100Hz as I wanted to retain some 'depth' to my surround since 100Hz is starting to become directional I wanted the surround effect to stay at the sides and back rather than at the front in my sub. I only have a global crossover setting in my Oppo, so I have to chose the best compromise.
Anyway, today I experimented with a phantom centre setting. This totally changed the sound, making it much more 'open' yet as my seat is dead centre it is near impossible to tell that there is no centre. Even sitting off to one side still sounds better (not that my OH would notice TBH
). I found that I could run at 60Hz crossover (40Hz started to sound 'muddy' again and the speakers sounded less punchy/dynamic). This worked better than 80Hz since I have a bit of a bass null at my seat and using 60Hz gives a more even response.
It really has made the system 'sing' and I listened to a 'Police' concert disc twice as it just sounded so good set this way. It seems a bit of a waste of a £600 centre speaker, but I'll look into trying to rig up a higher stand for use when I have my projector on.
It's threads like this that have made me reassess my set up (which I've become a bit disappointed with lately due to the colouration issue) so I'm grateful for the information within, even if it just spurred me on to try a few experiments for myself and let my ears (rather than THX) be my guide.