Most significant upgrade

I have been upgrading for the past two years. My experience upgrading requires that I provide you two answers. I submit speakers are the foundation of a system and you should build around them. I just added an Audio Research DAC7. I don't want to exaggerate, but it sounds like I have new speakers. The sound-stage has opened up and music is much more enjoyable. I don't know if the Rega Apollo is that bad or the Audio Research is that good. Adding the DAC has had the most significant impact on improving the sound "IN MY SYSTEM."

Would depend on your budget system room and listening tastes.

Beyond all doubt, room treatment.

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?

I think resolving this conversation will depend on where your son goes to college...if he gets into Bates with no financial assistance you may have to sell everything and buy a small table radio...he goes to a local state school and it's HIFI BONANZA TIME!