What did you add to your 2 channel system to do HT?

I "added" a soundbar. No interest in making a dual purpose system and a few hundred for a decent sound bar suffices for me. YMMV

If you are happy with it, then just use it that way, and enjoy! Sounds like you got lucky with the front 3 channel speakers. 

My HT receiver does 13ch with 4 sub outs, also has pre-outs for all CH, with the ability to mix/match. It also allows me to have different speaker arrangements for different inputs. Also a dedicated 2ch setup with or with out subs

If you can do anything like that, do the 2ch setup, with all the good gear, then have a separate configuration for HT, using your good amp for 2ch, with the internal amp doing the rest.  HT really sounds better with more than one sub. IMHO, HT needs more subs over 2ch. But I'm sure many will disagree. 

Finally got everything sorted on mine, so pictures no longer fall off the walls when watching action movies at reference volume. 

I wonder if the question of the asymptote applies more to to HT than to 2 channel listening.  I ‘m sure that a mega bucks processor et al will sound better for video than a Mid Fi AVR, but would the expensive gear be sonic overkill for videos?.  Even  the most glitzy sound effects can be competently handled by an AVR, and most soundtracks are meant to be background or ambience.  There aren’t to many Alexander Nevsky (Prokofiev) or The Last Waltz type movies out there, where the music was conceived as at least coequal to the video content.  Now if you watch a lot of concert videos, or Blu Ray’s of operas, you might prioritize the sound in a HT system.

@mahler123 I have a number of concert videos on DVD and Blu-ray.  A few of them have really good audio. Most don't.

You definitely wouldn't build a system like mine if your emphasis was on movies. Mine does OK for me, but it would be pretty lacking compared to a high dollar dedicated home theater. 

I need to get The Last Waltz.  Thanks for mentioning it. 

@mahler123 there is nothing to say that high end 2 channel sound and high end HT(or any level HT) can't coexist in the same room unless it is room treatments.  

In my system they coexist and the only shared components are the 2 channel amp and he front speakers.  

My 2 channel sound system stands alone.  No shared components when operating in 2 channel mode.

My HT system routes a preamp level signal to a second input on my amp to drive my front speakers.  So maybe the front speakers and amp cost 10x what the rest of the HT system costs, but they are there for the 2 channel system and certainly don't hurt the HT system, which sounds excellent.  So I don't consider that "overkill".  I use a Marantz Cinema 50 to drive the surround speakers and do the decoding for the front.  I have a large center speaker in the center of my rack that just sits there quietyly during 2 channel sound and is driven by the Marantz for HT.

I know my Marantz reciever reciever and 2000 lbs of floorstanding surround speakers and  subs would be considered mid fi by some HT enthusiasts but I find it to be pretty darn good.

Very happy with both systems.

Jerry