Hi Willy Martin,
I am curious why i would not accomplish the same thing from the headphone jack?
The headphone out jack is an unbalanced stereo signal. All the inputs on the T4S are balanced mono. These two signal formats are incompatible.
If you connect a stereo signal to a balanced input, you may hear very little, or just effects with none of the original signal. A balanced input will take the left and right signals from a stereo source and combine them with reversed polarity (this is how balanced connections work). The result is sounds that are panned in the center (typically, vocals, drums, bass) will be cancelled.
If you were to use a 1/4 inch (6.3mm) Tip-Sleeve cable, you might get sound, but it will only be one side (probably left).
If you were to use a 1/4 inch (6.3 mm) Tip-Ring-Sleeve cable, you probably will hear very little, or a distorted sound.
For more information see: Balanced vs Unbalanced
Is it because the Boss speaker would then be muted? That isn't a problem in some cases where i would basically just use the L1M2B2 as a PA and a monitor (when I have the space to put it behind me (which, surprisingly, is only about 50% of the venues I play)) and the Boss as a "mixer".. (As an aside: I actually use the S1 pro as a monitor through my T8S headphone jack...easier, for me, to adjust volume levels.)
The Aux input on the S1 Pro is designed to accept a stereo signal. If you connect the T4S/T8S headphone jack to the S1 Pro channels 1 or 2, with a 1/4 inch (6.3 mm) Tip-Ring-Sleeve cable, you will likely have problems. If you use a 1/4 inch (6.3mm) Tip-Sleeve cable, you will get only the left channel. If you are running the T8S mono, then you may get get sound but not both sides.
ST, IF you think this is okay to do (based on I dont mind if the Boss is muted), which cable would you recommend (from the headphone jack of the Boss to the t8s)?
You could use a cable that splits the stereo output from the Boss

to two mono outputs.

Then connect those mono outputs to two inputs on the T8S.
All set?
ST