If you want a tube preamp to drive a solid state amplifier, the thing to look at is the output impedance at 20Hz. What it is at 1000Hz is really not that important- most any tube preamp will have an acceptable output impedance at that frequency. Its how the preamp behaves in the bass that is important.
This is because most tube preamps have an output coupling capacitor that is part of the equation of output impedance. At 1KHz, its no big deal but at 20Hz that coupling cap becomes a major influence on the numbers and can cause a loss of bass impact.
To counter this issue, some tube preamps have a large coupling cap so they can drive solid state. The problem is, the bigger you make that capacitor, the more it can color the sound, even if its Teflon. The reason has to do with the inductance that the cap has which is something independent of the insulator in the cap. The bigger the cap, the more inductance (due to the fact that the cap is wound). This creates a tension in the design where the cap is chosen to color the sound the least, while at the same time not affecting the bass. It is at best a compromise.
A very small number of tube preamps have a direct-coupled output, which sidesteps the issue.
This is because most tube preamps have an output coupling capacitor that is part of the equation of output impedance. At 1KHz, its no big deal but at 20Hz that coupling cap becomes a major influence on the numbers and can cause a loss of bass impact.
To counter this issue, some tube preamps have a large coupling cap so they can drive solid state. The problem is, the bigger you make that capacitor, the more it can color the sound, even if its Teflon. The reason has to do with the inductance that the cap has which is something independent of the insulator in the cap. The bigger the cap, the more inductance (due to the fact that the cap is wound). This creates a tension in the design where the cap is chosen to color the sound the least, while at the same time not affecting the bass. It is at best a compromise.
A very small number of tube preamps have a direct-coupled output, which sidesteps the issue.