i use FXpansion Etch a lot... nice options on it.

How about just automating the low pass filter on the Quadcurve? Or is that too simple?

Bare in mind R-Mix is no longer supported so a changing the plugin for something else (if possible) is probably a good idea.
You should be able to use it in X3, although I've had limited success myself.

Hi Boydie.
 
I've used the built-in channel EQ to great effect with a sweeping high or low cut but if you need to make quick filter sweeps, that doesn't work so great but I've used this plug-in with excellent results and it's free!  They also do a paid for version with more options for little money but the free VST worked for me!
 
http://minimalsystem.com/Store/index.php?route=product/product&path=61_62&product_id=50
 
Then there's this one which a lot of dance producers swear by and has been used a lot and now that it has dropped in price, is not a bad buy at 30 Euros!
 
http://www.sonalksis.com/creative_filter.htm
 
The best part about both of them is that they are incredibly easy to use.
 
Good luck and let me know how you get on.

Waves MetaFilter is pretty cool especially if you want to do things like automate a HPF on a kick etc.