I believe it's more than the amp selection here that points to brightness.
I offered up the Rotel as a BTW mention. I had the rotel playing a pair of VR4 JR with a BAT VK5i preamp. the VR4 JR speakers have a lot of top end energy and regardless the SS amp I tried with them I had to keep the back tweet turned off... even with a BAT vk500 BP. there's not a bright bone in the 500's body!
Cabling played a big part in taming and straightening out brightness. the other major item in controling top end energy was power filtration.
I had a pair of Monitor Audio Gold 60s + a Krell KAV 250 + Sony 444es HT receiver as a preamp for a time then too. this was the most analytical, sterile, yet clear as a bell sounding rig I've owned. The cabling was crap. there was no power filtering going on, and there might even have been a ground loop issue for all I knew then.
ADding another upscale proc/pre helped. in the end I sold it all off... save the Sony receiver. I feel now though, had I investigated cabling and conditioning sooner, some of that rig might have stayed around lots longer... conditioning andd cabling are just that big a help.
However, given the choice between either getting another amp or changing out speakers, actually I'd do both eventually, but first I'd try something lots cheaper... a power filter like a PSA upc 200 FOR A COUPLE HUNDRED or so, instead of major changes first... and/or a power cord swap out. Then maybe do other big changes. that might well be all you need for now.
Addressing the incoming power in any rig is usually a great idea... though many attribute power line issues to a lack of component synergy or poor componnets instead.