Answer
Hi Luis, thanks for getting in touch. The current Silent Brass Studio pack, the STX-2 is designed to work with the latest PM-X Silent Brass Mutes.
It's technically possible to use the STX-2 with the older PM model mutes but bear in mind that the Brass Resonance Modelling (which makes the tone through the earphones or headphones sound incredibly natural) won't be enabled and that you might experience some distortion (hence we don't support or recommend that combination).
We have a pack which includes the latest Silent Brass Mute and Studio Pack for Flugel Horn: