A lot depends on the M2 involved and the equipment you have. After all there are pressure washers and pressure washers. We have a fairly big pressure washer set up and some decent rotary surface cleaners. Personally however especially on Riven Slate I prefer to use a Rotary with Soft to Medium Nylon Brush, some high alkaline and a decent wet vac. Alternatively why don't you do a test area with your pressure washing set up.
Kev Martin
Marble Life Ltd