Tubes Asylum: Running Mullard EL37 Tubes in 6L6GC Headphone Amp by rlawry

Does anyone have any ideas why my Ampsandsound Forge amp, which is designed to run in triode mode 6L6GC, EL34, and 6550 family of tubes, will not run NOS Mullard EL37 tubes? I bought a matched pair and they do not even light up even though the 6SL7 input tube lights up normally. According to my past experience with EL37s in speaker amps they ran successfully in place of 6L6GC tubes but not in the Forge, even if they have the same pinout according to online sources. When I try other 6L6GC variants such as the GEC KT66 and new production Tung-Sol 6L6GC-STR tubes before and after trying the EL37, they run normally but the EL37s never even light up. The EL37 seller says he tested the tubes fully prior to sending but I guess I will have to send them back. Any ideas, anyone? Thanks.