Would a quad of NOS JAN/Philips 6L6WGB be good in a Silverface Bassman? Someone told me they would reduce clean headroom-I'm not really sure what that means. Also,for the phase inverter,I've got a JAN/Philips 12AT7 in there at the moment. Would a Mullard CV4024 be a worthwhile upgrade? How about GE12AX7's for the preamp spots? Any advice appreciated.