Hi tubelss,
Short answer: get Class A amp or full body midrange speakers like Harbeth
Long answer: Just sharing my exact same experience at this very moment, the problem you're looking to solve is 'synergy' not just an amp. As I aged from late thirties to early forties these past few years, I've come to exact same problem that I get 'listening fatigue' much sooner with my B&W 805D3. I used to be able to listen for 1hr now 15mins start to feel long. 'Bright sounding' is just one aspect, the problem is much broader that just that. To avoid writing a long paper, below is my bullet points on my journey of progressively honing in on solving system synergy.
1) Speaker cables, its the biggest impact assuming your system is already at certain above average level. Audioquest Rocket88 subdues brightness very well but you'd lose details. Ping me if you want my set. I'm currently using a 6 AWG cable handmade by someone in USA from ebay that's shockingly good for $400.
2) Finding the right amp. Big difference between amps that make 100W at 8ohm & 4om vs 100W @8ohm / 150W @4ohm vs 100W @8ohm / 200W @4ohm...etc. After trying NAD Master, Mcintosh MA5200 a few other small brand stuff, I've settled on Marantz PM-10.
3) DAC, I've recently upgraded to Gustard X26 Pro
4) Interconnects
5) USB cable
Amp alone wont solve your problem. Good luck with your search for syngery journey. Just make sure the speakers are really ones that you want to stay with before you splurge 10 grand.