I play both French horn and marching mellophone, and was wondering if the Yamaha SB3X-2 French horn silent brass pickup mute would work for the mellophone, so I don’t have to buy a second mute end.
The Yamaha website lists the Silent Brass SBX Series, including the SB3X-2, here:
There, there is a link to a Compatibly Table for the SBX Series. Right up the top of this page it says:
The Pickup Mute is not compatible with Piccolo Trumpets, Alto (Tenor) Horns, Baritones, Alto Trombones, Bass Trombones or Marching Brass Instruments.
Also, down the bottom of the page there is a Please Note section, the second dot point says (quite explicitly):
The Pickup Mute is not compatible with Marching Mellophones, Marching French Horns, Alto (Tenor) Horns, or Baritones.
So unfortunately, it looks like the SB3X-2 won't work for your marching mellophone.
According to the Yamaha website the SBX series is actually discontinued, but even the new SBJ series of pickup mutes apparently doesn't have a model that is compatible with marching brass instruments.