I'm seeing everything from 3000-8000 as the appropriate Primary Impedance on the OPT for a single ended 6v6 amp. Seems that higher plate voltage calls for higher impedance? Many say "5000." The OPT I bought a while ago for a "Champ" has 7000 on the primary (4 on the secondary.) I use a 125D Hammond for a homebrew SE 6v6 since I added a switch for 4 or 8 ohm speakers. Problem is, I use connections resulting in about 8500 on the primary. This because one data sheet I saw said 8100. Today I tried the cab's 4 ohm on the current "8" setting which means 4100 was on the primary instead of 8800. It was a little brighter which is good for this amp. Not much though. So what's the REAL impedance I should use on the primary with about 300V on the plate and 280 on the screen in class A SE? My choices are 2800, 3400, 4100, 5800, 6400, 8800.