Would plugging a cable into the send jack be safe? The reason I am wary of all this stuff is because I read that it can damage your head to be on without being plugged into a cabinet.
The preamp should be fine with a cable plugged into the send jack, as long as the cable does not get shorted.
For tube amps, the power amp has an output transformer to match the output impedance of the tubes to the speaker. If no speaker is plugged in, the output transformer will build up energy and eventually fail usually taking out other parts of the amp, causing costly repairs. Because the preamp output is much lower, there is no need for a transformer, so it should be fine.
In the setup you are talking about there is no input to the preamp, there should be no output on the send jack (other then some noise). If your preamp is quite you may not need to cut it out of the circuit with a cable. If your amp has a side chain (blender effects loop) then you may not be able to completely cut the output from the preamp, but most amps with a side chain should have pretty quit preamp.
The power amp section is not the same as the preamp, even when there is no input there is some output (how much will very with amp design) and there maybe damage to the transformer, especially if left for a long time with no speaker load.
Hope this helps.