If you want to TRI-AMP, that is a different amp for the Tweeter, a different amp for the mid, and a different amp for your subs, then you need a 3 way active XO.
What I was talking about was a way to use the internal (passive) crossover of the 9.0, and crossing over to subs.
If you set the NHT X2 to cross at say 70 HZ, it sends the 70 HZ and above to the 9.0's amp, and the below 70 hz to the subs amp.
OK, we know it's not a brick wall, it's a 24 DB filter but for simplifications my example above is true.
You can get more info from John Casler on the NHT X2. I have one and very much like it.
What this accomplishes is that your subs play the frequencies they are better at, the 9.0 no longer has to play frequencies it's not that good at, and it really cleans up the midrange.
Dennis Murphy also has a nice passive crossover design for the 9.0.www.murphyblaster.com
Tomorrow he will be getting my mini marble column 9.0 to do his magic on the XO.
Even crossing over to subs, you might want to talk to Dennis.
His nOrh 9.0 XO design is free and on his site if you want to DIY or have someone else build one for you.