Lukewarm heater output

5.3 in an ‘88 XJ-S, lukewarm heater output. Back flushed heater core, flows garden hose fully on with heater valve still in circuit. Engine temp consistently at bottom of “N” on gauge. New rad, hoses, thermostats (88C), coolant. Same temp output from heater whether in auto on manual full heat. Both heater hoses feel the same temperature, but cooler than rad hoses.
Anyone had experience with the coolant rails clogging or thermostats not blocking off bypass ports causing this?