cathode-follower tube preamps..

If you are trying to drive an amp with a low input impedance, and you want tubes then you will need one that has a cathode-follower or Mu-follower output.

However the amp has a balanced input so you might consider tube preamps that are balanced as well. I prefer tube preamps with a direct-coupled output in this case, as they play bass much better with greater extension.

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.