benefits of a phono preamp for me?

The main advantage you gain is that you will be able to use LOMC cartridges. Other than that, I like the idea of getting away from the "plug In card" that’s inside the 99, and getting a proper external unit. Love your system, keep in mind that the phono pre is the gateway to excellent vinyl sound, so spend accordingly. I use the DV20x2L w/Manley Chinook and I love it over the Clearaudio Maestro v2 ebony. Also, all the $200 to $500 phonos sound like crap and even up to a $1200 Musical Surroundings Nova, though the latter was a vast improvement, it wasnt until I went for a Chinook (at around 2k there are several brands to choose from) that my vinyl world really improved to vastly higher level of detail and "aliveness" the others lacked.

Matt

You have a very nice system and all components are in line with each other in terms of price and quality. I would not invest in a new phono-stage, just keep it where it is. They will produce diminishing returns until you start spending a lot of money. Your dealer can give you a couple to try at various price points and in the end you will wind up staying where you are most likely. What cables do you have?

I remember the 99’s phono board sounding pretty decent, and it’s supplemented by the very high gain of the line stage. Actually makes for a pretty good match. However if you upgrade to the next level of low output MC cartridge, like the next Dynavector up, then I think you’d benefit a significantly from an Ares. If you do go for an Ares, I’d also strongly recommend the Blue 1254 CineMag upgrade ($400) because the Red 3440 holds the Ares back a lot. A K&K Lundahl premium kit is another good option for SUT.

It’s not out of line to split costs between your cartridge and phono stage nearly 50-50.

@pacmi03 audio sensibility... 

Personally, I'd keep your current set up and be happy with it as pacmi03 alluded to.
If the buy bug bites and you must spend money then I agree with mattmiller's way of thinking.
I have an Allnic H-1201 phono (tube) preamp that I'm very happy with.