Don't run it without the power tubes. Simply removing them will change (raise) the operating voltages globally. Not good if there are other active sections, such as a tuner, in the unit.
Either do what Brian suggests, or find a tech that can pinpoint the summing spot where all inputs (including the tuner section) feed the output tube section. Then decouple, ground the output tubes' input point, and reroute the L/R signals to a seperate pair of output jacks.. which you'll have to install. Line level, if not available/acceptable at one of the stereo input jacks currently in the unit, can be achieved by reconfiguring the input (load) resistance of any available stereo input pair.
This whole procedure is pointless if you like the sound that the output tubes are generating, in which case Brian's suggestion is the only option. That'll leave you with not really just a pre-amp, but a fullblown amp that's been stepped WAY down. A resistive network can then be added to the dummy load to give you a line level signal. It's a trick used by guys to feed a seperate Leslie speaker from the internal speaker of a Hammond organ that has a built-in speaker/amp.
Unless you have a schematic or a knowledgable tech, don't fool around in there, as there are high voltages associated with tubes, and you'll be shooting in the dark.
Best, Paul