I’d maybe give the Krell more time to see if it settles in better as it burns in more, but if not the first place I’d look would be the front interconnects/cables. I wouldn’t pair any silver cables with older B&W Nautilus speakers and thinking that could be the source of what you’re hearing, especially as it existed with the Butler and got worse with the more transparent Krell. Plus, of your options cables are probably your cheapest/ easiest option to explore (especially if you’re open to buying used) and are obviously cheap/easy to ship. If I was you I’d try these Acoustic Zen speaker cables and see if you notice an improvement — you could try them with your L/R fronts and then separately with your center to gauge the effect on all your front speakers. I’m thinking they’ll have fuller mids and more refined highs that could be a better fit for your system, and if they don’t work out you can turn around and sell them at little/no loss as there’s a liquid market for used AZ cables.
https://www.ebay.com/itm/235430798208
Anyway, that’s what I’d do FWIW, and best of luck.