I'm late to he party, but here are my two cents:
Back off of the throttle a little bit!
Sequentially:
1) You are rearranging your room - big deal for the sound!
2) Changing the sub(s) is the obvious weakness in your current gear! Again big deal for the sound!
3) $1200 for a pair of Ultras is a great deal and you should have no problem selling them on Craigslist for $1200 (assuming they are in fine shape)
4) Replace the AVR, but know that Anthem is a waste of money - generally Anthem measures worse than the mainstream units such as Denon, Marantz, Yamaha despite the high price!
1&2) As far as I am concerned 1 & 2 are no-brainers! You might want to wait a week or so between 1 and 2 simply to drink in and observe the changes from your rearrangement of the room while running your original gear if you are the curious sort.
3) Normally I would say after the rearrangement and new (serious) subs, take time to totally reevaluate your system because you may well experience a totally fulfilling system. However the $1200 deal on the Ultra towers is compelling and I think it is a good idea to spend some time comparing the Ultras vs the Mirage when you can do it with so little cost (given you can sell the Ultras quickly at $1200). I have heard both and they are both very nice speakers. They are so totally different, without either one being wrong!
You do not want to change speakers and be that guy regretting how much you miss the sound of your old Mirages. Like I say both are good, but you want to take the time to validate your decision to keep or replace the Mirages before you do it! If you spend time in that room when you are not specifically listening, keep the Ultras running at a background level to give your subconscious "burn-in" time with the different dispersion of the Ultras. Often, that which is not familiar, seems, on first brush, wrong out of hand. Spend a couple of weeks with the Ultras then see how the switch back to the Mirages feels!
You do not need a center for evaluation of the speakers, just be sure your AVR knows you do not have a center (run the setup routine) and you will be impressed at how well the mains cover the center (I have found this true with any good speakers)!
FWIW, I love classical music (especially strings) on the Mirages and something like Chicago (the band) is better (IMHO) on the Ultras. I think that is simply a reflection of how strings radiate sound in all directions like the Mirages do while trombones and trumpets are definitely forward facing.
4) Changing the AVR is likely to make the least impact on your overall sound quality, and I would generally say it does not matter what you get as long as enough clean power is there for your specific speakers (neither of the speakers you are considering requires more than a solid AVR). However, it seems that you have very limited options on the placement of your sub(s) which means the ability to compensate for room modes is likely to play a significant role in your sound quality. Accordingly I would suggest you get an AVR with Audyssey XT32 (in other words Marantz or Denon - Denon is better value unless you happen across a great discount on a Marantz, but if you love the look of the Marantz, why not?). Be sure to get the Marantz/Denon Audyssey app for your phone (or tablet) for $20. This will allow you great control over how Audyssey is applied in your room. Many of us have found that running Audyssey below 300Hz is a very good way to tune the bass to your room and integrate the subs for a seamless handoff to the speakers.
And I'll sneak in 5):
5) If you like to sometimes really crank it, you could feasibly push the power limits of your AVR, so buying a 2-channel amp or three monoblocks for the front speakers and letting the AVR handle the surround channels is an option. If you are crossing to subs so that the AVR is not powering the bass of the mains (this usually works best even if your mains have great bass), I would suspect you will never run up against power limits, but down the road, one year of your $1500/yr budget would easily cover this! Monolith amps or Outlaw's monoblocks come to mind off the top of my head!
I have done this, not because I need to, but because I like the idea of my system just coasting along without ever working hard. Also consider that your AVR will continue to fall into obsolescence every ~10 years, but good amps should be good for 20 years or longer!
