Water for Partial Mash Russian Imperial Stout

Gonna bottle my first batch today and I want to pitch a Russian Imperial Stout on top of the yeast cake (used WLP 001). I want to do a partial mash, and my question is:

Do I need to be concerned about the water chemistry for the mash?

My recipe (4.6 gal batch) calls for 6.6lbs pale extract and for the mash, 6lbs of 2 row + about 3 lbs of specialty grains.

LHBS informed me that our tap water is on the soft side (Cambridge, MA). Should I try to adjust something for the mash, or will the extract take care of things?

Thanks

Soft water is [subjectively] good for a stout anyway, as long as you aren't going for a super dry, Guinness style. RIS, big and smooth - soft all the way.

So, no, I wouldn't worry about water salts. You need to have a good idea about what your mineral content is to begin with to do good adds, anyway.