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

What are you trying to get out of it? Your center channel amp is overkill! If you have a HT receiver, use the HT receiver amp for HT, then use your good amps for 2ch. For HT it is very important that all speakers are timber matched, as sound goes around the room the tone is matched just as the volume/db should be. You also have REL subs? Why not just use them and not another one? Feel like you got all the gear and just want to use it, you can most likely get better sound using less, and making it simpler. 

I have 2 separate systems, one for HT and one for 2ch. Neither does the other's job well. Will tell you when the HT is all setup correctly, it's awesome! Mine is better then most theaters. 

5.1 is good, when you step up to 7.2.4 or something like that it becomes magical. You can hear a door shut in space, as well where the plane is coming from, how high, is is climbing or diving. It trips my dogs out all the time. 

@mswale Yeah, I realize that the Adcom amp is overkill, but I had it laying around, so I'm using it.  My two-channel amp is a much higher quality Coda.

The Sonus Faber center speaker is actually fairly well matched to my Von Schweikert mains.  That was just dumb luck as much as anything.  I have an older center channel speaker by Von Schweikert and the Sonus Faber is actually a better match. 

I have two REL S/510 subwoofers that I use for two channel.  They are great for that purpose, but my larger SVS does better for explosions, etc.  It sits in the back of the room and seems to work well there. It does nothing when I play two channel audio.

I'm really not trying to improve upon the HT end.  Just explaining what I have and wondered what other people did.  I'm not a real home theater enthusiast. My two channel system is much higher quality that the HT gear.

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.