tkr, You seem to have an elaborate system, although you don't mention downstream components. But I get the impression that selling one of these 4 cartridges is not a matter of economic necessity. So, I'd say keep them all. Most of us nuts own more than 4 cartridges and think nothing of it. In fact, I cannot imagine paring down to only 4.
PS. If you do get a second headshell for your FR64S, try something other than the ones made by FR for the 64S. By all accounts, they are not so great. (I too own an FR64S but I have never played with any FR headshell; I am just repeating scuttlebutt.)
PS. If you do get a second headshell for your FR64S, try something other than the ones made by FR for the 64S. By all accounts, they are not so great. (I too own an FR64S but I have never played with any FR headshell; I am just repeating scuttlebutt.)