There is some very good info on the Marlin Owners (MO) forum pertaining to your REP vs JM stamp question:
Marlin Owners Forum.
I have a Marlin 336W that I bought in 2009 and it is JM stamped. I did have a Marlin 336SS (REP stamped) that I bought in 2010 that was a decent rifle (good fit and finish, accurate, nice walnut stock set). I sold it to fund another purchase, not due to any quality issue with the rifle. I did take a hit on price due to the rumor mill.
That said, there were some posters on the MO forum that had some problems with their rifles, after Remington (the Freedom Group) bought Marlin, due to quality control. The issues were probably due to moving the operation, new equipment vs legacy equipment, training curve for new workers, etc. In the past year, the rifle quality of those that I have looked at appears to be getting back on track. Others have reported the same on the MO forum.
Best suggestion I can give is to carefully look at them to make your own decision prior to purchase.