Upgrading my system

The fact that you're saying the sound is "nice" is a good start.  Perhaps you should consider putting some resources into maxing out what you have, such as cables or platforms.  Many would argue against this and there are many who would agree, but I have found that the right cables can make a good system great. What are you using now? 

Speakers are connected using Cardis cross link bi-wire cables and interconnects between components are Audioquest Big Sur. Think both are about right for the system. Should have mentioned also have Emotion turntable and NAD CD player (can't remember model number but about 10 years old) 

I doubt you would go wrong with the Harbeth. It is my favorite of all of the many LS3/5A type speakers that I have owned. This guy likes them as much as I do!!

I'm not familiar with the equipment you've mentioned, but I have owned a number of bookshelf speakers - Canton, Monitor Audio, and Vienna Acoustics to name a few.

I have the Harbeth P3ESR SE speakers in a room a little smaller than yours, in a nearfield setup (about 4 1/2 feet from my listening position).  They are detailed and have that luscious Harbeth midrange and dynamic, but easy to listen to even at that close distance if I decide to push the volume up.  They need some power and a subwoofer or 4, but they are hard to beat in that category.  I change out gear fairly often, but I don't see these going anywhere in the foreseeable future.

A friend had some in a well treated room closer to the size of yours where he was able to set them well out into the room with two subwoofers and they sounded amazing.  They have their limits in terms of bass and volume, so if you like to listen loud, go bigger.

Speakers are typically the thing that will change the sound of your system the most (for better or worse).  Good ones will reveal other shortcomings you may have in your system (amplification, sources).  If you want to stand pat with your system and make a meaningful change, I would start with room treatment before cables. 

I think of cables more as "do no harm" than a system "upgrade".  Not that they can't change the way things sound, they can and do, but my goal with cables is not to change the sound of my system, rather to make sure they aren't getting in the way of the music.  Room treatment on the other hand can have a significant positive impact on your listening experience.