Adding Home Theater To 2 Channel System


I have a two channel system that I am very happy with located in a dedicated listening room.  I am considering how to add home theater without giving up the two channel sound that I enjoy.  My main speakers are Avantgarde UNO’s (horn speakers - 18 ohm, 108 db sensitivity). Avantgarde does not make a center channel speaker.  I assume that I will have a problem matching any box speaker center channel speaker with the Avantgarde - my worry is incoherent sound.  Is this something I should worry about?  Are there center channel speakers that might be a good match (I have been unable to find any obvious candidates ohm/sensitivity match)?  Is it likely that I will need to find a new set of front speakers from the same brand as the center speaker?

The rest of my setup: Pass Labs XA 60.8, Audio Research Ref 6 preamp, SME 20/2 turntable, Lumin x1 streamer, Ayon CD player, REL 812/S subs (2)
chilli42
millercarbon is right, a good system does not need a center channel. Doesn't need rear channels either. It will need killer subwoofers.
I also have a 2 channel theater. Check out my system page and you can see it. Just hang a screen between your speakers and you are good to go. Not quite. It took me several weeks to get all of the rattles and buzzing out of the screen and it is a Stewart!
I'd replicate the advice/sentiment shared already and forego a center channel. If ever that route would have to be taken the only true solution would be 3 equal front speakers, period, but then you'd need a perforated screen + projector for that to make any sense. 

What I would do instead, and that wouldn't only serve Home Theater duties, is investing in a massively capable sub set-up, which is to say loads of displacement, headroom and no less than 20-25Hz extension (some may say honest 15Hz is required for HT to cover most, as in 99% of the infra-sonics found on the many Blu-ray/4K UHD titles, but a lower tune impacts the overall sensation of the bass - i.e. central to upper bass, and hereby integration in particular - so it's a compromise). This is MANDATORY for movies, but properly integrated (which goes without saying) will serve music reproduction as well. 

As for the particular sub solution (and ULF capabilities) to choose here, I'd forget about über-expensive subs and instead amp up their number for a DBA sub set-up, or go with 2 very large ones. If acoustic live performances are very much your cup of tea and form a reference for your home audio reproduction, I'd place the subs (say, 2 or 4 of them) symmetrical to the mains and coupled in stereo. If not a mono-coupled DBA placed asymmetrically may float your boat. Either way, never skimp on displacement and headroom, and get sufficient extension.  
Thanks to everyone for the terrific and consistent feedback.  It will save me a lot of time, money and disappointment.  Your advice on this is invaluable.
Good to hear. 
If you end up trying rears again just use any old speaker you have sitting around or get something cheap.

 timbral matching the rears is a waist of money unless you are really into it. 

Highend rears is a waist too. If you run high end rear speakers you will feel obligated to power them right and use good wire etc, and it all ends up not being worth it. I think for 99.9% of us we would rather spend that money improving our two channel. 
A screen is the best way to go. If you really want the least possible compromise use an acoustically transparent screen, one designed to have speakers behind it. This way you can put whatever acoustic panel you want behind the screen and it will still work. Or for more money a powered screen you can raise and lower, but those come with their own set of problems. 

In any case, done right you will wind up with four subs in a DBA and this alone will elevate your stereo like you won't believe.