When my birds get old enough to sex i seperate the roos, put them on a much higher protien and fat diet, and by 16-20 wks old they are plumped up just fine and the extra fat in the diet makes them more tender, I have heard many questions regarding tough and tender topics, my view is it is mainly in the kitchen where this is done, most folks i know are not very good cooks and tend to over cook poultry, which will make even the best chicken hog feed. marinading your meat is mostly a crutch some cooks use as an excuse to tough meat to mask their skills.
if you have confidence in the way these birds are raised and processed, then take the same care in the kitchen your efforts will be more rewarding.
AL