Need help Setting X-over on amp!!!!

Initially, use your adjustable crossover on your eq to split the usable signals to your sub and your highs.  If your eq's crossover is on-off selectable, you can just use the amp's onboard crossovers.  Actual crossover points vary according to taste and listening volumes.  Hi bass volumes usually require a lower crossover point.  You will want to gap the hi-pass xover point a little so as to not make the hi's work so hard. 

In either case:  try setting the sub around 80hz, and for the highs set xover point around 125-150hz at the amp if not selectable at the eq.  Also use your individual frequency points on the eq to help tailor the sound.  Whatever the "correct" settings are, the goal is to eliminate distortion at the hi-end of the sub and at the low-end of the highs. 

Clipping as in, past the point of human audible distortion and the sound starts cutting out?  Your amplifier gains maybe set too high.  Also make sure there arent any user selectable boosts on the rear of the eq that could be causing this.  What type of head unit are you running? 

You say that this is only a problem with the t400?  how do you know?  you adjusted the crossover points individually by connecting only one amp output from the eq at a time?  you are sure the sub is not distorting?

Sounds like you have gain set too high somewhere else as you didnt mention these other electrical components previously.  The sub is most likely distorting as well - but not to the extent that you can hear on the highs.  But if you say its not....... 

Assuming the h/l converter wiring is correct, check for output gains.  Set them at the just-above-minimum setting.  Also, goto your balanced output processor and turn the gain down there as well. 

You will need to turn down all the gains everywhere to just-above-minimum and start tuning your system from there.  If you turn one or more gains down too much than you may not hear anything. 

Be careful with any signal processors.  They help tremendously with sound quality but can also overdo it - as what appears to be your problem.  Try not to have any one component on full gain and remember that too high of a boost on your inputs will clip and spike faster than you can say FRIED chicken.

Good Luck.