Receiver for pre/pro?

I've currently got a NAD T752 receiver and am looking at an outboard amp as an upgrade. I'm 80% 2 channel music but do use the system for 5.1 surround sound with movies too.

Thinking about an amp from Pass Labs either the X150 (2 channel) or X3 (3 channel to include center channel).

Looking for opinions from people who use or have used a receiver as a pre/pro and an outboard amp. Does music sound much better with this setup as opposed to using the amp in the receiver? Volume isn't a concern for me as the NAD is plenty loud enough.
As the previous posts indicate;much improvment can be had.---I would suggest, by all means do the 3ch amp upgrade.Much/most of the info comes from the center-- do all 3 channels.--- I might add the benefits will require a quality center speaker.---A $200 cheapie won't get it done here.
One more question on this subject. If you take a rather warm sounding receiver like NAD or H/K and match it with a brighter amp like a Adcom or maybe Acurus-Aragon, will amp's sound take over and you lose the warmth?
Well in my opinion--you wouldn't do this unless you had an amp you knew was much better,--in the first place. If you're just experimenting with something you already have,that is one thing.---I do again want to state the importance of the center speaker/ amp quality.There are so many less than stellar center speakers out there.
The reason I asked was that I have a 50 watts into multi channel H/K 230 and would like to boost at least the front three channels to about 80-125 watts. Maybe a used B&K. But if the H/K's warm sound wouldnt dramaticly change, something like the Acurus three channel amp that resells for a decent price used looks promising.
I have an very nice Ohm Acoustics center channel so I have that base covered.