How to tame a bright system?

Not familiar with your amp, but I auditioned the B&Ws you have and found them headache inducing. I think trying to remedy with cabling is merely damage limitation, and isn't going to solve your problem, especially if you're sensitive to those HFs. Some people love the 606 S2s, and I could see their appeal for some, but would need careful equipment matching. Sorry ☹️

Funnily enough, I did see a YT video the other day by GR Research who supply speaker kits for some brands/models of speakers. They supply a kit for your speakers which are claimed to tame that top end and some other idiosyncrasies of the 606 S2s. From memory, I think the kit was $250. Assuming you're in the States

I think cables are the wrong way to go, but if you must, mid-range Wireworld cables generally are pretty warm sounding.

You may find listening on the woofer axis to be more pleasant, as well as reducing the toe-in.

Finally there is room treatment. Taming the mid/highs will help the bass bloom.

If you can wait until you are in your 60's, the problem will solve itself, otherwise I would look into replacing the speakers.

Not the wire. Change the speakers. First find some with at least 92dB sensitivity that have the warm sound you are looking for. 92dB because this will give you a lot more amplifier choices, especially in your price range, that will also have the warm sound you want. Then you will find the Fire sound just fine.

Finally, put everything on Nobsound springs. Real nice improvement and with the added bonus they will allow you to further tune and refine to get just the balance you want. Nothing else out there even close, certainly not for the money.

I'd try some Kimber PBJs and some lower line AudioQuest Type 4. If that doesn't get you there, it's speakers or preamp time but I'd give the softer cables a try first.