Hey All -
As for the original problem that started this thread, ScreaminDaisy makes a good point that (if it is the amp that is bright) this is most likely a tube that has mechanically gone wonky or a bit microphonic and is amplifying the 'wonk' as the signal dies out. If it's vibrationally related and you have the amp on top of the cab, if you take the amp off the cab and run it and the noise is less of an issue or goes away, this indicates that potential mechanical/microphonic tube issue.
The tubes are always the first place to start because they all are destined to fail at some point and sometimes they fail within 2 seconds, 2 days, 2 weeks or 20 years - that's why companies selling tubes provide a warranty on them. Rather than change out the whole set, a calculated approach to troubleshooting is suggested. More on that should it be needed.
Like many well known and often appreciated British amps, the Stiletto's (Ace, Duece and Trident - all the same preamp circuit) will likely come off a bit bright to many at the start, especially with music store settings (12:00 noon on gain and tone control settings).
One quick place to start that is settings based is trying lower wattage settings and Tube Rectifier settings. This allows you to turn the amp a bit more and the natural clip and overdrive lends a warmth and roll off of the extreme high end.
There are some great suggestions here like being aware of your speakers and cab and whether they are bright or neutral or darker in general. An amp only ever sounds as good as the speakers its being played through so experimenting with other cabs or speakers can tell you a lot about how your cab is affecting your sound overall. Vintage 30s have a bit of a reputation for having a peak in upper mids and low treble that many perceive as straight treble brightness. Plugging your amp into known darker or neutral speakers can be a great reference and learning experience.
The other excellent suggestion is that brightness will be more balanced as the amp is turned up and bass frequencies are essentially being supported and amplified at those higher volumes. The amp is more full at higher volumes and higher frequencies get smoothed with more bass in the mix.
Finally, not being afraid to use some more extreme settings to the low side, especially on Treble and Presence and even the Mid control which affects a good amount of the upper mid and low treble region. It may not seem comfortable to have treble, mids and presence set below 9:00 but in the end, if the sound is there, it's not about the settings, it's about the sound.
There's a few other things you could try but this could be getting a little lengthy. If you're interested, feel free to contact me on the forum via PM to discuss further. Hope this helps.
