Basic Questions

If you are happy with your present system you don't need to change anything.  The reason to add a pre/amp or integrated is for the HT passthrough feature.  You could then run your two channel sources through the new pre/amp equipment instead of the AVR.  For the HT front left and right you would need a pair of interconnects to run from the pre/out L&R of you AVR to the new two channel set up so you can watch movies.  Then you would connect your turntable, streamer/DAC etc to the analog "in" connections on the new equipment.  Not sure about Yamaha but some AVR's convert everything to digital.  So you could run HDMI from your Oppo to AVR for BlueRay movies and use the analog out on the Oppo to play CD/SACD on the new equipment.

The new amp would be a two channel amp and power the front left and right speakers unless you really changed things by getting an AV controller with a new multi channel amp like for example: McIntosh.  A very expensive solution.

Another option to try and improve the sound quality could be to get a more expensive AVR with an in home, risk free trial.  Hope I have answered some of your questions.

I think I'd look for an integrated stereo amplifier with a home theater bypass that would allow it to function as the power amplifier for your home theater setup while operating independently for stereo listening.

I tried to go that route with a Cambridge Audio Azur 840A (just didn't like the sound) and then Krell S-300i, but found my Pathos Classic One Mkiii to be superior to the Krell so am using a custom speaker switch that allows my speakers to be connected to either my home theater receiver or the Pathos.  It's not elegant, but it's functional.

I'll keep it simple. The levels of equipment performance are: Receivers, Integrateds, Separates with stereo amps, then Separates with mono amps. The better your preamp is the better your system will be. 

P.S. The Parasound A21 is a fine amp. 

There are a lot of questions there.

If you want high quality two channel sound the best way to get there is through a completely different 2 channel system. AVRs do not create high quality sound… too much stuff in a box. If this is not the route you want to take, then @mceljo +1 This would be the first step. Or better yet a separate preamp and amp.

While technically a preamp is sometimes not required, the reality in +98% of the time they are (especially without huge knowledge and carefull choices). They bring the magic. They take the really small line level signal and ever so carefully amplify it and very carefully enable you to control the volume and source.

Ideally you want carefully chosen high quality components… for instance:

$1K streamer, $1K DAC, $1K preamp, $1K amp, and $3K speakers.. Double the investment in each component (all) and you will get a strikingly better sounding and far more satisfying system. But, well chosen… they must be chosen to match your taste and be synergistic with the rest of your components.

I have been improving and enjoying high end 2 channel sound for fifty years. My components are now at the $22K, $17K, $17K, $34K, $32K respectively. You can see my systems… including my home theater system under my UserID. The most rewarding system I have heard.

I would rather separate my 2 channel system and build it in a large walk in closet, than combine with my home theater. Carefully building a system to your space is critical. They have completely different requirements and the compromises of the HT generally make it really difficult to achieve high quality sound. Movies need less quality and more bang because you are distracted by the video.

So, I guess, if you are interested in putting together a high end system. I would recommend Robert Harley’s book, the compete guide to high end audio to give you the lay of the land.

@ghdprentice +1, someone gave me that advice when I was looking at tube pres with HT bypass. The light bulb moment hit and I focused on the BEST MCH system I could have in my HT and moved my 2 CH to my desktop. More satisfying and less money that trying to have 1 system do both things well. I still listen to two CH in the HT but only about 10% of the time.

OP, if you want GREAT MCH SQ on music and movies I can post what worked for me, let me know. I found it much more rewarding to focus on power and front end components.