In that case, it's the Theil EQ box that's driving your power amps. The output impedance of your preamp is irrelevant as far as your power amps are concerned. The Theil EQ box will eliminate any direct interaction between preamp and power amps. I expect that the EQ box, having a solid state output stage, has a pretty low output impedance (have any specs on it?).
I think you'll be fine with this configuration.
I think you'll be fine with this configuration.