Upgrade path

You should listen to your  ProAc D2R in your system first, then tell us which way sonically you’d like to go.  You may find you have enough satisfying bass, or not.  

Some augment bookshelf speakers with subs, but integration can be challenging.  If you have the budget, floorstanders are generally superior at higher costs.  In addition, depending on how low (bass) the floorstanders go, adding subwoofers can both extend to bass lower frequencies and support the floorstanders bass to midrange (fills it out).  

When building a audio chain, it’s usually centered on the amplifier or the speaker,  For example, I demoed many of the best systems at audio shows and stores to find the sound I like, then went out to purchase the speakers within budget.  In my second system, I wanted to hear tube midrange magic so I started with a flea watt 300b SET amp, then purchased a high enough sensitivity speaker that matches.  

ProAc D2Rs (arriving soon

How would you know what you are missing if the speakers haven't arrived/?

Once they do, you can do two things: listen and enjoy your perfectly capable system or start tweaking. (Yes, you will start tweaking)

I would wait with a sub until the ProAcs are dialed in. I would go with a different amp.

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.