We've taken a few elk of all types, cows, spikes, raggys, 6s and 7s. It seems they've all been pretty good.
However the age of a bull sure could affect it's tenderness and does. I too agree hard running game affects it as well.
But I believe the number one reason for tough and gamy meat is how long it takes the animal to expire. If it dies fairly quickly, then great ,if not, then adrenaline and such gets through the system and man can that effect the meat. In other words if it takes an animal 15 min. or more because of a marginal shot, (bow or rifle)or over an hour before it dies, this is when I've noticed the tough meat syndrome. So an older animal with a marginal shot just compounds the problem. A younger animal with a marginal shot and a passing of time could probably take a little more time.
In your case, a 5x6 should have been excellent table fare if disposed of quickly. Do you know how long it took for his bull to expire, or if it required some tracking and what not after the hit? elknut1