If you have a direct-coupled output on the preamp the output impedance will not vary with frequency (with most tube preamps its usually the bass where this is an issue).
If you have a 10:1 ratio at 1000Hz but only 5:1 at 20Hz, you will hear that as a loss of bass impact. This is because there will be a slight loss of output voltage in the lower frequencies.
If you have a 10:1 ratio at 1000Hz but only 5:1 at 20Hz, you will hear that as a loss of bass impact. This is because there will be a slight loss of output voltage in the lower frequencies.