For the price of the monolith THX-365C ($450) there are a number of other better center channels (preference score in parentheses):
- Elac Uni-Fi 2.0 UC52 (5.0)
- Infinity RC263 (4.7)
- Monolith THX-365c: (4.4)
Also, as others have noted, it would seem that the KEF Q350 (5.7) is a great option for center channels at the moment because it is less than $500 for a pair ($250 each). Accessories4less has it for even cheaper. It is coaxial which is a benefit for center channels (according to others who know more than me).
To get a better scoring center channel than the Elac you need to spend triple the $$$, so its not worth mentioning an upgrade. To get a better bookshelf speaker to use as a center channel (compared to the KEF Q350) you need to spend $750 on the Polk R200.