Upgrade path

Great start.  Give it time and get very familiar with what you have.  Learn to optimize the room and system setup (speaker placement is huge), then if you still want to upgrade something, evaluate what your complaints are and go for the low hanging fruit first....it's a system, there's synergy, preference, and it'll only be as good as the weakest link.  In the end, it should always be your preference that prevails.  Enjoy...this can be a life long journey!

I’ve a similar sized, dedicated listening space. Based on my experiences, I’d sound treat the walls and ceiling, as well as add a pair of subs. Knotscott has offered good advice.

Get all of your components. Hook them up. It will take at least 200 hours for everything to be broken in. The sound at first will likely be trebly and flat. Speakers must break in, the cones must get exercised... but every electronic component must go through breakin as well.

During breakin you need to get the setup right. The sound you get will be strongly dependent on the speaker location... the toe in, distance from walls. Then the room may need dampening. 

I recommend not even thinking about upgrades for many months... six months. 

In the mean time get a copy of The Complete Guide to High End Audio by Robert Harley. This will start helping you understand the elements of sound, the relationship to components. And the high end speak. 

Then start thinking about upgrades. 

First, take delivery of your speakers, assemble your system, then listen to it at length - a couple of months at least.

Carefully matching good amplifiers to the needs of one's speakers' is always a must. If you find that the end sound produced by the ProAcs pleases you and fulfills your expectations, then taking that step may bring to your system some of what you will find missing in your auditions.

Optimize your room.  It will be the best bang for the buck and will make future electronics upgrades even better.