Hi paxromano,
Generally a foam mattress does best with a firm evenly supportive foundation where the foams in the mattress do the “work”. In some cases however … an active box spring will also work but as you mentioned this will affect the feel and performance of the mattress. If this improves the feel and performance of the mattress … there is no reason not to use it as long as it is still in good condition.
The boxspring should have no weak or soft spots and you can test this by pressing down firmly in all areas of it with either your hands or your knees. It also shouldn’t have any areas that are sagging and you can check this by putting a straight edge or using a tight string across the surface. If your box spring is 14 years old … I would personally not take the risk that it would affect your alignment and support on the mattress. The box spring will affect your support more than your pressure relief although it will have some effect on both. Box springs are also not designed to hold slats in a stable way and I would not recommend it. Some people have put plywood over the boxspring to make it firm and solid but I am not a great fan of solid surfaces under a mattress (unless it can’t be avoided) because it can restrict ventilation and increase the risk of mold, mildew, and other “undesirables” but I tend towards the cautious side with this and there are others who think it’s fine … and in most cases it probably is as long as the wood is dry.
The best way to change the comfort level and pressure relief of a mattress that is too firm is usually by adding a topper. In some cases a more flexible base can be effective but this generally works best with a thinner mattress or where the flexible base is part of the design of your sleeping system.
There is more about the difference between foundations and active boxsprings in post #2 here.
Phoenix