It is hard to give you a definitive answer to this. The short answer is that yes, this will work, but the SQ may be compromised and it is possible that the preamp could be damaged. Preamps are designed to amplify VOLTAGE, which is what the input stage of a power amp requires. As such, they typically have output impedances in the range of 1K and up. Dynamic headphone drivers are analogous to loudspeakers and require CURRENT, and operate at impedances of 600 ohms or less. Many preamps CAN supply adequate current drive for good headphone performance (conversely, many headphone amps have sufficient voltage gain to function well as preamps, such as the X-Can), but it is difficult to determine this without knowledge of the technical specs. for the preamp in question. In general, preamps with a lower output impedance are more likely to be successful in this role than those with higher impedance.
The biggest concern however, is the possibility of damaging the preamp. This can occur because the headphones may draw excessive current from the output stage of the preamp (particularly low-impedance (<120 ohm) and low efficiency 'phones), leading to premature failure of the output devices (op amp, transistor, etc.). The highest risk for this would be in preamps with higher output impedances and/or those designed without an output buffer. The output buffer circuit does not amplify the signal (unity-gain) but rather helps to stabilize the output impedance, which is constantly changing based upon the frequency of the signal: under low-impedance conditions the buffer supplies current and when impedance increases it supplies voltage. This can be accomplished with transistors, op amps (some specifically designed for this purpose), or tubes (generally a cathode-follower configuration). The objective is simply to supply the power amp with as clean and stable a signal as possible.
Unfortunately, I am not familiar with either of your preamps so I cannot speak to whether or not yours specifically would work in this application. Hopefully someone with specific experience with these units will chime in.
In theory, one could design an impedance matching transformer for this purpose. The principle is similar to the OP transformer in a tube amp. The primary could be in the range of say 5K ohms with a 600 ohm secondary, and would connect between the preamp output (primary) and headphone (secondary). Unfortunately, I'm not aware of the existence of any such transformer or commercially-available device which does this (it would seem this could be a high-demand item around these parts however!). I readily admit that I'm no engineer, so there are quite possibly practical considerations that make this more difficult than it appears in theory to implement. Maybe someone around here is adventurous enough (financially and otherwise) to have such a transformer custom-wound and put this to a real-world test!
One other possibility is to sell the preamps. There are a lot of fans of both Quad and HK and a lot of vintage gear sells for suprisingly high prices. I don't know whether or not your pieces happen to be models that are highly-sought after, but further research is (IMO) definitely worthwhile-start with ebay and Audiogon.
Sorry to give you such a noncommital answer to your question, but I hope this at least sheds some light into the complexities of what you are trying to accomplish.
Cheers
