Not sure what to tell you, since the stock speaker is designed to be flat so that it represents the amp and cab models accurately. But, that's not to say there aren't better choices out there. Eminence makes a speaker designed for modeling amps that might go better with your amp than the stock speaker. This might, however, be the exact same speaker that is in your amp. A lot of manufacturers will use eminence speakers and relable them. You might also want to look into an Eminence Delta Pro. This speaker is similar to the discontinued EV12M (not sure if that's exactly right) that was worshiped by some.

I'm not tying to pull you away from Lats' choice of speaker, just giving you some more options.

One more thing you might want to try is, use an EQ unit in the amps effects loop. Check out the line 6 forum. There are several tricks like this that people use to get the best out of their modeling gear.