HiroPro,
The EXBAC network on our 340SE/SEC crossover is not designed to be a user serviceable option. If a customer orders a 340SE "center", EXBAC is enabled by default and the speaker is tested as such.
That said, enabling / disabling this option is certainly not what is causing a lack of clarity on some source material at extremely loud volumes. Gov has had these speakers for a long time now and I do not recommend opening them up and disabling EXBAC as that will not resolve the issue.
130 watts is not that much power, especially when considering that you are using auto room EQ. For example, even something as minor as a +6dB boost of EQ in a specific frequency range will change the available power in this frequency range from 130 watts to a measly 32.5 watts...
The key factor here is that it sounds fine at all volume levels -except- when played extremely loud and with only certain source materials... As an example, auto EQ will often try to compensate for the large dips caused by floor bounce, which are typically found between 200 - 600Hz (lower end of the vocal range). These dips can be huge so even if we assume that your receiver is only applying a small 6dB boost to this range (which is small in comparison to how deep these dips can be), at 12 feet back from the speaker, combining the wattage capability of your amp with the sensitivity of the speaker, your amp will be clipping in part of the vocal range at anything over 92dB. A level which I would consider to be loud, but quite tolerable. When class D amps run out of juice, they tend to sound very edgy and harsh...
I would recommend the following:
Turn auto EQ off then determine if you still hear the harshness using the same source material and same volume level. Granted, without auto EQ it might not sound as good overall, but we are trying to determine what is causing this so that I can make a qualified recommendation as to whether or not the forthcoming STC will fix the issue...
Using an SPL meter, measure the volume level at the listening position when you first start noticing the issue...
It is also important to consider that since this seems to only occur with certain source material, it could very well be an issue with the source material itself. You could also try swapping the center with one of your mains – just to eliminate the possibility that it could be a problem with the individual speaker.
Hope this helps!