benefits of a phono preamp for me?

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.