Richard,
I don't know if there is a difference between the III and the IIIx but here is my response based on the information from the Monolith III and that the two Models having the same crossover frequency:
I would not use the Monolith crossover with your Ascents. The reason is the crossover frequency is much lower on the Monoliths at 125Hz where your Ascents are crossed over at 280 Hz. Using the Monolith crossover would put a much higher demand on your ESL Panel. Using the Monolith III crossover would send more bass information to the ESL Panel, possibly overdriving it at higher volume levels.
Sam