What to judiciously invest in next to improve SQ?

[zambie]

You said "this is an expensive hobby" and you didn't mention a budget for improvements. That makes it hard to offer suggestions.

You identified 8 areas but not a way of conceptualising those areas. I'd break things down into 2 broad areas, the system and things you can do to improve that, and how you make the most of the system and what you can do to improve that. The system refers to your components, software, and cables while the "how you make the most of the system"  area concerns things like vibration control, room acoustics/electronic room correction, vibration control, and I'd probably consider power cables and power conditioning as part of this area. I'd leave speaker cables and data cables/interconnects as part of the system area.

You can improve anything but it makes sense to start with whatever you think is most holding your system back at the moment. In my experience that's usually one or more things that will help you make the most of the system you've got rather than something in the system itself. Then it comes down to what you can do with whatever budget you've got. If you've got limited budget then I'd simply start with something that will cost you time and a bit of physical effort and nothing else, room and speaker setup. Start experimenting with where you put your speakers and listening position because that can make a really major difference to your sound quality. Even if you're planning of adding physical room treatments or electronic room correction at some point in the future, you'll get better results with those things if you've got the speakers and listening position set up well in the first place. Read your speaker manufacturer's recommendations for setup and start with that. One of the things I find really important is symmetrical placement of the speakers with respect to the side walls and the wall behind them. Each speaker should the the same distance from the wall behind them at the very least and, if possible, the same distance from their nearest side wall though that isn't always possible. Try to ensure that the distance to the wall behind and the wall to the side aren't exact multiples of each other. If you can place each speaker one quarter of the width of the room from the side wall you can cancel a lot of the room mode associated with the width of the room which can help a lot in the bass region. Where you sit will also affect the bass response you hear so experiment a bit with moving the listening position backwards and forwards a bit from your starting point in order to find the spot with the best/smoothest bass response. Since you've got electrostatic speakers, try angling the speakers towards the listening position so that the plane of the electrostatic panel points towards the side wall first reflection point. Electrostatics have a null at the side in their radiation pattern and angling them  so that the plane of the panel is pointed towards the side wall first reflection point will minimise the strength of the side wall first reflection. You're probably going to have to  experiment a bit to find the best location for the speakers and listening position but it's worth the time and effort. Good setup can really bring a system to life.

And you can then put the money you've saved by working on room setup rather than buying/upgrading something aside and save a bit more so that when you come to make your next improvement you've got a bigger budget for that.

To be honest - I wasn't focussing on the price side of things, but more along the lines of what are the pieces in the audio chain + components that impact SQ more than others based on experiences that all of you have. That said I like how you've conceptualized things. It is difficult to say what's holding the sound back when one's not aware of what other things are capable of improving - hence the question.
Surprisingly enough - my Martin Logans aren't the electrostatic kind. At this point - I'm quite certain my room situation is quite far from ideal and is definitely impacting the overall sound output.Point taken on finetuning speaker position and angle. The piece I can't seem to wrap my head around is that certain audiophile recordings e.g. some of Chesky's productions are absolutely great and have a great sound stage compared to a lot of "mainstream" recordings of rock bands - which seem to be much more of a "recording" than an actual presence if you know what I mean... whether that's a function of the quality of the recording or the setup or both is something i need to comprehensively figure out.