In short, I have an excellent preamp (ARC Ref 6). Rated voltage is 2V RMS (balanced) to a 200 ohm load with a max of 70V RMS. This is just way too much for headphones, but it wasn’t designed for them.
First, the output voltage rating is based on a load of 200,000 ohms ("200K"), not 200 ohms.
Second, the 70 volt number is irrelevant, as the output voltage will be a function of the gain of the preamp, which is not particularly high; the volume control setting of the preamp; the output voltage rating of the source component; and of course the volume of the source material at any given time.
And while the combination of those factors may or may not result in too much voltage into the headphones, depending on the particular headphones, the much more significant problem is that the output impedance of the preamp (nominally 300 ohms single-ended and 600 ohms balanced, and probably significantly higher than that at deep bass frequencies) is much too high to drive almost all headphones with decent sonic results.
What you would need to accomplish what you are asking is some sort of active buffer stage that would provide a very high input impedance and a very low output impedance. Which might be doable, but it would probably be simpler and better to just buy a headphone amp and connect it to the tape outputs/record outputs of the preamp, as Clio proposed.
If you decide to do that, while I don’t know what the unspecified output impedance of the preamp’s tape outputs is, to minimize the possibility of impedance incompatibilities I would suggest choosing a headphone amp having an input impedance of at least 47,000 ohms (47K).