OOPSIE! Though it was your sig that listed a 6500. What other Macs have you got to do a baseline test of that CRT? The following applies to just about any other Mac you may have on hand.
the oopsie part:
Stop wasting time with IIsi testing, I see you've got a 6500. Use that to get the monitor up and running at 640x480 in MacHz(Don't recall offhand) and at 16" Mac Standard. If your 6500 can put those pixels on the CRT, the IIsi can as well.
If the 6500 can't do it, all the IIsi tinkering in the world is a lost cause.