I remember hearing the AR18 speakers and thinking they sounded a bit aggressive - though I appreciate you have the BX version so may sound quite different.
Agree with Paul7777x about the suspect bass/mid driver surrounds. Foam rubber drive unit surrounds don't usually last much beyond 15 years, IME, and I suspect start to change compliance long before visible deterioration is noticeable (if tsurrounds stiffen, I'd expect that to limit bass performance). How long ago was the current foam rubber surround installed? And was it by a competent DIY'er? I'm not convinced the Sony amp is the main cause of your dissatisfaction, though it could be. Early Sony amps I've heard were quite polite sounding.
FWIW I dropped off a pair of my speakers at WA 1 feb 21 to have capacitors replaced, service and a couple of upgrades. Due workload and component supply issues and covid their expected 4 week turnround was 3 months
That’s not too much of a surprise. There have been lots of manufacturers struggling to fill orders recently. There is genuine chaos in parts supply, especially in China (hardly surprising considering how many are made there).
I’ve had one experience with Wilmslow and they were mighty helpful.