The Cygnus modeling is legit when comparing it to real amps. The top end sizzle in 21 is similar to (if not a little less than) the sizzle I hear from my tube amps, so the modeling is getting more and more accurate. If you’re going from FW15 to FW21, try a making a preset from scratch and see how you feel. Keep in mind that a big difference since FW15 is that the Master Volume now has a taper authentic to each amp, so try reducing it or bumping it up to get the power amp sitting where you like it. The Master Volume and Presence controls are probably the two most important knobs to pay attention to when dialing in a tone. Get those right and the amp practically tweaks itself.
If you don’t like top end air, use the high cut in the Cab block to shave off what you don’t want just like using an EQ in your DAW to shape the top end of a real recoded amp if it’s needed.
For the amps I’ve tested and matched with FW21, these were the closest:
Brit 800 to my ‘77 2204
PVH 6160 Block to my 5150
Solo 100 Lead to my Soldano Avenger
Dizzy Silver to my Diezel VH-2 (the Fractal actually sounds better than my amp)
USA Lead Mid Gain to my Mesa Mark IVa
Matchbox D30 to my Matchless C-30 head
The Dual Rec models need a little more help to match my Rev F, Tremoverb, and Multi-Watt, but they’re still in the ballpark after tweaking the model’s GEQ section a bit which isn’t a big deal.
All that to say, tweak FW21 like a real amp and you’ll get more realistic results.