How to remove hoses that are clamped to a water manifold in a hot-tub
Probably you should just replace all the hoses as well as the manifold, if the hoses are so set/stiff with age that they cannot be moved. That tends to be a step on the path to cracking and leaking. One big repair .vs. lots of smaller repairs over time.

You could try dunking the whole mess in hot water or using a hair dryer/heat gun to warm them up and make them more pliable.