Audiophile grade passive impedance buffer

I think your only option is a transformer unless you want to go for active buffer.

Question is whether your preamp's output impedance is resistive or not.  If it's resistive and your amp's input is also resistive you'll at worst suffer loss of gain. 

If it's a tube pre with capacitor outputs chances are it's capacitive and output Z goes up as F goes down.  One possible solution is to increase the capacitor sizing.  More uF will reduce the output Z in the bass region.

I should add that otherwise, imhifiman is correct.  An impedance buffer is the solution.  You'll trade impedance for gain, but at least it should ensure a flat response.