Switching from solid state to tubes

Nothign wrong with the idea, but the dynamic range and power requirements for film are a lot more than for music.

The biggest issue you would face is the disparity between your mains and everything else when listening to movies. You could alleviate this somewhat by setting the L/R speakers to "small" and routing all bass through a sub.

tube pre-amp with SS amp(s)

A VAC PA 100 offers 100 watts with taps for 2,4, and 8ohms.  Used, should be around 2k.  Sound is adaptable via choice of input and driver tubes.  It doesn't run very hot either which is a bonus, not much more than a solid state amp.  You can do better for a lot more money, but with that said I kept the PA 100 and sold the 10k plus VAC Phi 110.  All in all a well built workman like amp that offers a lot of tube magic and flexibility for the money.  
It also doesn't go up in smoke like an ARC amp when a tube goes south.  Single ended inputs only.

Theater definitely seems like the afterthought here.  To have really good sounding theater you need the LCR all the same speakers and same amps.  You could go with a smaller Sonus center that is voiced the same. 
But, your post here is for tube amps.  If you go with a 2 channel tube amp, you will further hurt the theater sound.  As long as you are aware of this shortcoming.
My solution should be cheaper than the rest:  Use your Bottlehead as strictly 2 channel with the Sonus Fabers since you love the sound.  Buy 2 more Emotivas for your right and left for theater.  The Anthem is a beautiful piece and can easily run 2 systems for you in varying ways.  (just make sure it can pass the analogue signal to zone 2).

Good advice here on testing to see how much power is needed, and as well, alternatively keeping Flicks and mucic separate if a tube amp for the mains is desired.

Based purely on content of the two formats, music and films, my question on this maneuver is why fix it if it ain’t broke, with respect to HT?

The proposal for using a tube amp to run the mains, depends on the Anthem’s ability to pass ALL signals digital or analog to a secondary zone. Most all procs will pass analog but not digital to zones other than the main one.

If your streaming is analog prior to the Anthem, you are in a less complicated scenario.

Regardless, if you proceed with throwing in a tube amp, merely reconfiguring the mains speaker cables from what ever amp to the Tube amp and perhaps a touch of altering the levels on the Anthem is all that ot be necessary for tube amp enjoyment. This is, of course with the caveat you have a tube amp capable or maybe better said, compatible with the SF speakers needs.

True too, you may find as Atma said, you might not need all 200w the HT amp is developing, to satisfy your speakers needs. 60w glass amp is as good a place to start as any, albeit there may be more contestants up around 75 to 100 in push pull affairs available. So, hide and watch the listings for what’s what in tube power amps and costs, or dive right on in.

Good luck.