Amps or Speakers which one to upgrade next

From my experience, I have found that improving my system was best served by working from speaker to source component. My system improved most from isolating my speakers from vibrations. There are a wide-variety of products that fit that category. In my case after auditioning various products I settled on Star Sound Technologies Audio Points and Mapleshade Plinth speaker stands. Good luck with getting to your audio goal.

artemus
I also have a pair of Watkin GenFour speakers, amazing music out of them. Every time I am at their shop Bill Jr and I have the most interesting conversations. Great people to deal with. Did he play the Boz Scaggs CD in his listening room?

Options

1 Change your speakers. It won't be easy because no loudspeaker can be much of an improvement over what you have. They can be different though.

2 You already have a great amp. Again there's nothing much better out there. Just different, you probably don't want to go back to solid state.

3 Its certainly worth experimenting with sitting/isolation as both your speakers and amp are susceptible to resonance effect.

4 Check out good quality recordings/masterings. Your system deserves it.

When you're already near the summit, there's not much left to go. A second pair of ears is also essential at this rarefied level to prevent hallucinations.

A good recalibration trick is to listen to a extreme low budget system for a day or so. You'll soon fall in love with your current setup again.

many hold to the idea that front end is most important as in "garbage in, garbage out"

I disagree. The most colored component is the speaker. It is the starting point for upgrades after acquiring a competent set of components before it.

Proof? take some very high end speakers and power them with a wide variety of amps and front end components. Even though the sound will change the overall characteristics will remain the same. Then audition a wide variety of speakers and the sound changes drastically. That means you should find speakers you like and build a system around them. If you pair a $100K speaker with a $1000 amp it will be far superior to a $100K amp driving a $1000 speaker (assuming the amp is capable of driving the speakers)

as far as investing in music, unless you are vinyl only, with the proliferation of high quality streaming sources these days music is very cheap

Before you spend a penny on new equipment or room treatments get this:
https://www.musicdirect.com/accessories/IsoTek---Ultimate-System-Set-Up-Disc-CD
No affiliation.
I was really surprised what a difference repositioning my speakers made ad the unconventional position that sounds best. I would never have them set this way if I didn't hear the difference using this CD. But it may be mostly because I have a very asymmetrical room. Still the difference is at least on the order of a speaker upgrade or more for me.