I need to make a few large format (50 by 70 inch) prints for my gallery and was wondering about using an Arches paper or the Epson Somerset Velvet. I currently use epson premium luster and love the Dmax but want a "nicer feeling paper". How does the dmax of the somerset paper compare to Epson Luster and does anyone have any experience with any of the arches papers?

Neither of those papers will give you the highest dmax available from matte media.
I would look at the Innova and Hahnemühle papers for highest densities.
Still, none of them will reach the dmax of the photo surfaces you are familiar with, it's a different look.
Tyler

Also take a look at Moab Entrada. Good dmax and color gamut from a fine art paper.

The highest Dmax available in a large mat paper is from Crane Museo Max, my favorite paper for many reasons.