The first thing to reduce brightness is to position the Forte’s to fire directly forward to start off. The more you tow them in until they’re facing directly at you, the brighter they will be.
Second, I’ve tried several tube amps including the McIntosh MC275 VI and Rogue ST100 with Dark upgrades, a solid state Enleum 23R, and a hybrid Van Alstine 600R, but the best amp that I tried has been the First Watt F7. The F7 adds more body and warmth while reducing the brightness. You need to be careful matching a preamp to the F7 because of its low input impedance (10K ohms), so the preamp should have a low output impedance (preferably <100 ohms). I’ve tried 3 preamps with the F7, Holo Serene KTE, Holo Bliss KTE and a Van Alstine FETValve CF and they all worked beautifully with the F7. Another post above by dsper mentioned he’s had excellent results with the F7 as well.