I used to use triple crown senior (now blue seal SBP 14-10 is the same thing) for my hard-keeper 25 yr old arab. He has done really well on it and eats much less than previously while keeping much more weight on. In fact, I never need to worry about his weight anymore :)
One thing that I always recommend is to feed a supplement containing fenugreek. It is a natural ingredient that acts as an appetite stimulant. Without it, my guy would only pick at his hay, and this was why he couldn't keep his weight on. He now eats all of his hay. At one point, I tried to see how he would do without the weight gain supplement (as he was in fine condition) and found that probably the most important thing about it was that it caused him to eat his hay. When he was off the supplement, he started to loose his appetite. I think that this is often over half the battle with a hard keeper.
Hope this helps!