How big should center spkr be to set to "large"?

Yes it is a good starting point but lets look at just why that is?
I feel it was simply for the benefit of speaker manufactures, can you imagine small speakers that had good WAF if Lucas choose 60hz, 50hz? It is simply my opinion but I think that on one hand it is good to have a standard but they should be honest and claim that those numbers are more geared to the minimum acceptable performance and if you have speakers that can handle more.....by all means take advantage of the better performance.

"CD, do you know the reason for the THX 80Hz crossover standard?"

Drcruz - I've never considered why THX selected 80hz, seeing my system wasn't built around THX standards. I would suggest if its important, check out the website for THX or Tomlinson Holmans site to see if he explains his conclusion. IMO, considering I choose this as a STARTing point, vs. 250Hz or 45hz, it doesn't matter.

I'm glad to see the large setting works best for you system. Enjoy!

"Yes it is a good starting point but lets look at just why that is?"

I would assume it would have something to do with system integration...

"I feel it was simply for the benefit of speaker manufactures"

Conspiracy Theory?? : ) Just joking.

"I think that on one hand it is good to have a standard but they should be honest and claim that those numbers are more geared to the minimum acceptable performance and if you have speakers that can handle more.....by all means take advantage of the better performance."

I couldn't agree with you more. That why its a staring point. If the sub integrates better with the speaker performance at a lower cycle, then great. If you have satallites on all channels, then 80hz isn't where you wanna stop. But if it integrates at a higher cycle, then set the crossover higher. Its all about system integration, IMO.

Drcruz - Sorry I missed your other question.

"Is 80hz the point where sound is no longer omnidirectional?"

I can't say 100% its 80hz, but its in this region and lower, bass does become more omnidirectional.

Good Point CD, I pretty much feel the THX is for the "Big Box" Retail crowd and anyone with more than a basic knowledge of HT and equipment would dismiss it as only being rookie relevant.

I don't yet use a sub, and since my front L/R as well as the surrounds have deep bass extension, I can say that even if I did have a sub, I would still set the fronts and surrounds to "large".

Why you ask? Well, I have done some experimentation with watching movies and switching between sub "yes" and "no" settings. So, when I set the sub setting to "yes", the LFE is ostensibly directed to the non-existent sub. But guess what? Even then there is copious bass coming from both the front L/R and the surrounds. This is noticed in music, as well as ambient noise such as street sounds and industrial plant type noises.

This is not unlike the difference I noted when switching the center channel setting between large and small, again with the large setting preserving more ambient and realistic sounds.