Phono preamp upgrade price/performance

Separate PS can definitely improve sound. I would suggest that a reasonable price point is around same as cart, so ~$200 with the 2M Blue. Doubling that to $500-ish may still get you a bit more refinement, thereafter it becomes a question of what else will be the weak link/limiting piece. If you think of ratcheting your system up gradually, you may consider making the PS your best piece for now, then I would suggest something like $1000. Lots of options. 
Whether you want to stay MM only, or also leave the MC door open is another consideration. I think most >$500 PS are combined MM/MC (there certainly are exceptions).

From what I read on-line, the Arcam A15 is a one-piece unit that can do just about anything in the digital or analog world, provided one limits oneself to an MM cartridge. That's asking a lot from one chassis. So it's quite likely that you would experience a noticeable upgrade in sound quality by adding an outboard phono stage, even an inexpensive one. There are many good candidates. That is provided your TT, tonearm, and cartridge have more to offer, as well. Spend less than $500 on an outboard phono and budget a few hundred more for a cartridge that outperforms the 2M Blue, and you might be happy.

Schiit - Mani 2 or Skoll