Optimum primary impedance??

[dynaman1]

Gonna make some different P/P bass amps with some spare transformers I have. One huge output has a center-tapped primary with the option of using 3K3 or 3K8. I'll be using a pair of KT88s and my plate supply will probably be around 550VDC. I'm guessing the differences are probably negligible, but was wondering which load would be more ideal. Obviously, I need low end.

Thanks

[FYL]

Datasheets suggest 4K5 @ 560V, 3K8 would be closer, but speakers aren't fixed resistors, so...
Why don't you try both primaries and pick the one that you prefer?