Which custom FW is required for the model you descibed?
-- SimonSays
The official Mutable Instruments site that still hosts manuals / docs and original firmwares has a list of alternative third party firmwares available for each module.
Link here: https://pichenettes.github.io/mutable-instruments-documentation/third_party_firmware/
The alternate Marbles FW is called Resurrection, and it has the "hidden" t mode available. The firmware has a host of other upgrades and some of the stuff available is quite complex. I haven't yet dabbled into it, but it's something I'm planning to try on my Marbles.
Sadly Mutable Instruments closed shop so a clone is your best bet. Good luck with your modular journey!