so here is my question, I have Dynaudio 240gt 2-way components and a 4
channel MTX 4300 amp to power these. I also have an external Audio
control 2xs xover that I may or may not use.

I have the subs and sub amp figured out but am debating on the front
stage options. I have a hatchback and am omitting the rear speakers,
don't care for the fill.

I am planning on bridging the amp to get enough power for these, and
just using it 100Wx2 at 4ohm bridged.

I know this amp may not be in the same class as the separates but it's
all I have for now and I was always happy with the MTX quality, though
I may upgrade to a Zapco or Mcintosh if I ever get the itch bad
enough.

That amp is fine. It easily does more than its rated power, which should be
plenty for those speakers. There's no reason to upgrade.

Question is:

Are there any benefits to using an external xover vs the one built-in
on the amp?

No. I'm assuming your internal crossovers are configured for your needs.

The question is sound quality? Is there any diff?

Nope. An active crossover is an incredibly simple circuit. The tough part
is actually in the power supply. Piggybacking on the amplifier's hefty
power supply saves space, adds simplicity, reduces noise, and is simply the
better option.

Obviously if I add an external xover I am introducing more cables and
that's not good but I would think that the Audiocontrol stuff would be
better sounding than the built in Xover which I can turn off. I also
like the idea of having it set w/ a chip rather than a know that may
move and is set by my ears and not to a knowing frequency.

You WANT to set it with your ears. The frequency markings on the amplifier
are inaccurate. That's good. You don't want it to sway your judgment.
Your ears should be the deciding factor. Anyway, the differences in sound
quality are nonexistent. The audiocontrol units use a state-variable scheme
versus the typical sallen-key design that just about every amp manufacturer
I know of uses. The difference? Fewer active components. That means
slightly lower statistical probability of failure, all else being equal. In
other words, it doesn't really matter!

Pardon my being so verbose. It's just that we have a few people in here
that insist there are differences, despite the fact that tey've never
designed or even constructed any of the components we're talking about here.