So....looking at your gear, I would not say that the yamaha is inherently the weak link. Yamaha makes decent integrated amps and compared to your other equipment is not out of line. It is a $4K integrated.
I would suggest Hegel as an option. It is a lovely line of integrated amps but the H390 or H590 would be steps forward. This a neutral and resolving amp that I think would be a good fit with your speakers. It is extremely clean and low distortion.
If you want to try tubes, Octave makes an amazing integrated amp. I mention this partially because I listed a used unit like 10 minutes ago but they are highly resolving and the super black box will assist in driving any speaker. I was surprised at how detailed and resolving this amp was and I think tonally will match well.
AVM makes some stunningly good integrated amps. The Evolution 5.2 is nice, the Ovation 6.2ME is better. There are great reviews of each in Positive Feedback and Stereophile. These are well made units that are a little to the warm side of neutral but are extremely resolving. The Evolution 5.2 is my internal benchmark amp and what I listen too most frequently. I also have a Hegel H590 here and a variety of power amps.
Musical Fidelity makes wonderful integrated amps but they have been impossible to get lately.
Ayre Acoustics makes awesome units that would be an A+ match. They have a pretty broad range to the price. I have heard the AX5 Twenty as I had one in on trade. Awesome integrated. I have not heard their newer, less expensive units, but would be truly shocked if they were anything less than fabulous.
This is $6k to $15K in terms of price range with most in the middle. All will be better than what you have. It is just a matter of what you want to improve will dictate which direction you go.
Full disclosure, I am a Hegel, AVM and Musical Fidelity dealer, I have a used Octave V80 SE and Hegel H590 for sale. I have no affiliation with Ayre other than being a happy former owner.