My first concern would be connectivity. You mentioned movies, TV and gaming. What are your source components and will the Wiim Ultra provide the same connectivity you enjoy now? This is where AVRs have an advantage over streaming devices. Wiim does make good products for streaming audio if you are not a fan of Denon's HEOS.
What the 950H lacks is pre-amp outputs, otherwise all you would need is a decent amplifier. People often recommend AVRs because of the value they offer. It is not uncommon for an AVR to have specs that are just as good as separates for around the same money or less, so it's not unreasonable to choose an AVR and then add external amplification, unless you need the space savings.
The X3800H is a decent AVR with full pre-amp outs plus it has independent subwoofer outputs which is preferred to tame those room modes. It has 105W into 8 ohms vs 90W for the 950H, so a small increase in power but not that significant. If only using 2 channels you gain a little headroom because you aren't taxing the power supply as heavily as when driving all 7 channels.
Depending upon how loud you like to play movies, you may find the X3800H is enough to give you that little bit of extra headroom, and the independent sub outs could potentially improve the bass response depending upon how the room behaves. The 1723 THX Monitors have an incredible amount of dynamic range, so they can take advantage of extra power if you want to push them hard. I would see nothing wrong with adding a good 2-channel amp if you find that an AVR doesn't quite cut the mustard, as they say. You could try the X3800H as an upgrade and if you feel some extra power is still needed, add a good class-D amp when budget permits. There are several good options available.