The tubes don't have to be matched.
I can use up to 4X6AS7's or 2X6336's or 8X6SN7's or 6X6BL7's and 6/12/25V driver tubes with my GR Super 10.
I have an OTL amp Glenn built for me that can run 2X6AS7's or 2X5998's and has the 5998 mod.
I can run low impedance Grado's with the 6336's or mid/high impedance cans with the other combinations.
I'll let Glenn advise you on the caps etc.
I hope this info helps.