Are There Improvements that Can Be Easily and Quickly Made Without Buying More Stuff?

start w the Jim Smith book mentioned above…it has many…. free…ideas 

You have a really beautiful house. Looks like a wonderful view.

I don’t think you need to know what is wrong… or what your objective is. By trial and error you can find out what it’s capable of. As soon I have optimized all wires (which of course is an expense)… then I go after the venue.
 

The answer to your question: all the things you say you cannot change (in your system description). Placement, floor and wall treatments. These can make large differences.

Is moving your system to a dedicated / or at least acoustically better room a possibility? That much glass and open flooring are the first thing I would go after. You can see my system and see what I did was incrementally work on these things.

@mikelavigne 

Wow!  I did look at pictures of your room.  I appreciate your advice!  And I will follow up on it.

i spent 9 months treating every surface of my listening room through trial and error.

the cost was about $350 worth of cloth and thumbtacks, and about $600 worth of Aural T-fusor diffusers.

these changes were the biggest performance boost i ever experienced. this was after living in my purpose built room for 10 years somewhat frustrated i could not get things to sound better. since i did this in 2016 i have been completely satisfied with my room and system.

you can see pictures of my room in my details.

in the context of my room and system it was couch change.

Yeah…I like these kinds of ideas!  I’ve already made 5 - 6 upgrade purchases this year, so it’s time to step back to try some tweaks.  It’s easy to try things and if they don’t work I can go back.

I don’t have the opportunity to listen to better audio systems.  So I don’t know exactly what I’m “missing.”  I do hear live music occasionally, so I have that in mind as the highest level of achievement.  When I listen, I think “does it sound like it could be live…or is it flat like a recording?”  
Another area to explore is the adjustments available on my subs.  I’ve worked on those and have difficulty judging optimal “phase”, but bass is among the best sound of my system.  I turn it down so I don’t notice it per se, but when there’s strong bass it sounds great. I don’t hear any difference in different DAC settings.

I think my sound is excellent right now.  Of course listening to Radio Paradise different recordings sound better than others.  I get very close to “live” with percussion instruments like a wooden block.  Generally base and sub sounds are excellent.  I can’t say that many vocals sound like the singer is in the room.

I won’t repeat what my current system is, but I have updated my profile.

Your listening room is large, but populated with many reflecting surfaces.  Perhaps moving to a near field listening setup would be useful.

Speaker placement absolutely..the Room - moving things around, room treatments using stuff you have like bookcases, window treatments...lots of isolation devices are also often just stuff you already have around...

Speaker placement ala Get Better Sound by Jim Smith.

I'd say that this is a huge open-ended question. Perhaps if you tell us what you don't like about the sound of your system that will facilitate better answers.

I'll suggest that you try positioning your speakers in many different positions, angles, distances from the rear wall, sidewalls, corners, etc. Perhaps you have already done this?