Most significant upgrade

Your system looks pretty darn good already. What will change the way your system sounds the most would be changing speakers - not even talking better or worse, but that a change of speakers and placement within the room will make the most significant change by far. Now if you love your speakers and are asking how to make the system better centered around your current speakers, that would be a different question - and I would probably start with the amp that really matches well with your speaker of choice. And your analog source would also make worthwhile improvement - some will argue between the importance of source versus amp/speaker, but they are all important. But like said, if you want to change the basic sound you are getting, speakers first IMHO.

I do think room treatment is very important, especially so with handling of bass, and can have an impact on imaging too if you deal with side reflections, etc, but I view room treatment as an IMPORTANT tweak to optimize performance, whereas changing speakers truly changes the basic sound signature of the system, room placement and treatment optimize what the speaker is capable of along with the right amp to do the job.

So the starting point for me is, do you want to keep the speakers you have and build the system around them? Or do you want to find the best speaker for your taste (you might already have it) and start the improvements after that? OP, where do you stand on that?

Jim, what brought the Harbeth to mind? Excellent speakers, but what are you thinking that makes you feel you might want to go in that direction? A very different speaker than the VS - also excellent. I think Inna's comments are a bit overstated (a lot?) but not atypical of the audiogon commentariat. What could be said of your TT is that is more than good enough to show improvements in tonearm and cartridges. But if significance is what you are looking for, it is speakers that will significantly change the sound you have now, though an SS amp would probably be better suited to drive you ML, not an easy load for a tube amp.

I too love the Harbeth sound, and if you take the manufacturer's word for it, and a large number of users, it sounds pretty darn good with a wide variety of amplifiers. It seems you have some pretty large group of devotees in the Harbeth, Vandersteen, and Merlin camps - and Maggies too (though WAF might be lower than VS). Like I said, everything is important, but the speakers are the basis of the overall sound of a system IMHO, and I think many others think the same way on system building hierachy - though in a vinyl front end, I do think there is a lot of importance and differences to be had with the quality of TT.tonearm, cartridge combinations - less so with digital in my experience.

Shadorne has a very accurate take on the Harbeth. If I were not "addicted" to my Merlin, it is the speaker I would most highly consider, probably HL5. I suspect I would also like Maggies very much, but my amps are just not well suited to drive them.

Inna, are you suggesting Texan's are straightforward, or that their opinions oversized?

I think the point of all this is to motivate us to go to RMAF and have a great time debating till the sun rises:)

DItto, very good advice, and one rarely taken, even by those pimping lot's of money into their systems - I suspect many don't know how good their systems can really be - sometimes WAF affects this important parameter:)