If the dialog is too low in
level, just turn up the center channel volume - don't worry that this isn't what your calibration settings came up with. Also, some movie soundtracks literally weren't mixed well.
But if it's the
sound quality of the dialog that makes it difficult to understand (for all movies), then the speaker itself is most probably at fault. And judging by what this
page says about that center channel, I am 99.9% sure this is the problem. You don't have to spend $300 to get an improvement - don't take this personally but even a $100 center speaker from any major
speaker-only manufacturer should provide a huge sonic improvement.
If you're on a tight budget & you don't mind a
slightly large (ahem

) center, J&R Music is selling two different Cerwin-Vega models, the lively
E-Series for only $115 (way under its MSRP) and the smoother sounding
Classic center for only $120. Both are more efficient than average & the 816 should not have any problem driving them. C-V is coming out with smaller centers to accompany these so I guess this is why they're being sold so cheap now.