IMO, the M1 ADC is a fine converter - in fact, I like it better than several of the commonly used outboard ADCs available.
I personally do not believe it's worth adding a stock SBM-1 in front.
I ran that same setup briefly. Try it out for a while and see what you think of it. Then, I think the next question is:
Given the budget available to me, what will have the greatest positive impact on the sound of my recordings?
IMO, the prioritized answer is:
[1] mics
[2] outboard preamp
[3] outboard ADC