I don't have an answer for your main question (What is the best developer for Agfa APX 400?),but just wanted to correct what Sam said about the effect of dillution above.(posted 8639 days ago)The higher the dillution,the slower the developer action of course,but this doesn't result in finer grain.On the contrary.especially with developers like Xtol and D-76,it will give you slightly increased grain.This is because dillution reduces the solvency of these developers, and extended development time does not fully compensate for it.
In your exemple,the advantages of dilluted Xtol are better,gradation,better shadow detail,slightly more sharpness.The penalty is slightly more grain,which is quite negligible at 1:1.That's why 1:1 is the favorite of many.For maximum shadow detail,1:3 is best.
I had 1 or 2 caked Xtol packages,but otherwise i did not have a consistency problem.