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

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.

Mtrot, I know the reason you get better bass w/ subs set to "YES" (even w/o a sub). I was doing a google search for '"phantom center" +debate' and found out (at the AVS Forum) that whenever you downmix a Dolby Digital 5.1 audio signal, to any configuration other than 5.1 audio, that the DD player or receiver will automatically turn ON The "Dynamic Range Compression" (DRC). Meaning: Subs OFF = DRC ON; Surrounds OFF = DRC ON; Center OFF (like my setup) = DRC ON.

I did experiments on my system to confirm this. "Master and Commander" and "War of the Worlds" REALLY demonstrate this auto feature.

Drcruz,

I am not getting BETTER bass with sub set to "yes" in the absence of a sub. All I am saying is that in DD and DTS movie tracks, there is a lot of deep bass in the fronts and surrounds that is separate from the LFE bass, which is mostly rumbling sounds. Now, you might not notice this if you are running satelites for these speakers.

This is why, contrary to what some recommend, I set all my speakers to large in the setup menu. Like I said, I get more ambience and "presence" set that way. Now, when I then switch back to sub "yes", I just get a bit more of the rumbling LFE type bass out of the front L/R.

With respect to dynamic range compression, I don't think my receiver works the way you describe. The dynamic range control is called, "Night Mode", and has to be selected with the remote or button on front panel. When I set to sub "no", the Night Mode does NOT come on, unless I turn it on.