Help! How can I set the bias on Golden Tube amps?

The new Golden Tube amps arrived with an upgrade for the original stock 5881 output tubes of Svetlana SV6L6GC tubes. The driver stage tubes are RCA 6SN7GTB. The manual specs call for adjusting the bias voltage to 635 – 640 mV DC in the output tubes. The manual says to not set the bias voltage over the 640 mV limit. Two of the six output tubes have such a high initial bias (900+ mV) that it is impossible to adjust the pot down to the 635-640 mV level. The pot adjustment does not allow that much adjustment. Any suggestions about what I can do?