The BASE1L/CAB1L and the Legacy #990 command system have exactly the same track output signal driver circuitry, either of them will output the same level of TMCC track signal.
I agree with the previous post, stop screwing around programming the transformer! Set the track voltage to 18 volts and use the CAB1L remote to operate the locomotives. This should be a no-brainer, don't make it more complicated than it is. For command/only operation, I actually just use the Lionel Powerhouse 180 bricks, no mystery, if the switch is on, there is 18 volts on the track.