The Martin M600 set does sound somewhat better on my Shark: a little brighter, less muddy hard-strummed chords. May experiment further with Oasis brights and Worth clears. I'm guessing there is much more variance from sample to sample when you're dealing with inexpensive factory ukes. For example: With my handy dandy digital postal/kitchen scale, my black Shark weighs 465 grams/16.4 ounces. I weighed a white Shark and got 530 g/18.7 oz. That's a huge sample-to-sample difference in two theoretically identical soprano ukes. (Both were the current version with plastic body and plywood top.) Still, a perfect instrument for keeping under the car seat or giving away to a friend.
Postscript, written 4/28/18: After giving the Martin 600s time to settle/play in, helped by storage of the instrument under my car seat, I think the Martins sound even better. Sometime in a few months, after I finish a cross-country move and have access to my spare strings stash again, I may try some other clear fluorocarbon strings on this thing ... the Worths or Oasis, most likely, since I like them and have some. No on would ever confuse a Shark or Dolphin with a high-grade instrument, but for a dirt cheap beater or starter uke, it sounds and plays better than I ever expected. But get one from Uke Republic or Mim. They do a setup on everything they sell, and it makes a difference.