Pictures speak clearer than words...
I don't run any fuses in any models & the one pictured above has 6esc's under the hood.
Are you connecting all your esc's to a bus bar to power them?
Every bared wired you strip, do you solder the ends so there's absolutely zero chance of a single strand of a frayed wire going into a Sabertooth to create a cross polarity?!... that'd be my immediate determination of what happened to you.
It's grounding out somewhere... do you have a wire or a shrink tube covering that you might have nicked with the soldering iron causing an exposed wire?
Maybe you had a bad DE Sabertooth... won't know till you try another one... but 1st, triple check every wire - every connection.
I've never powered up a Sabertooth with the rx leads not connected... don't know what might happen, if anything bad could happen
...might the Sabertooth configure center point wrong & cause linear motors stall at end of travel? Are your Firgelli actuators in the center of their travel?
These Dimension Engineering Sabertooths have a built in thermal protection, meaning if you work the esc too hard & overheat it; it will timeout, cool off & work again. If you cross polarity the + - or ground out, they're smoked!!!