Maybe...
The reason to load a tube amp is so that an oscillation, or transient has something to get dissipated into. If there is nothing, the amp can develop a fatal over voltage in the output transformer.
So, if the pre is a transformer coupled affair, a load would be advised to protect the output Iron. If it has no OPT, then not so much. There is often a very light load in a cap coupled amp to make sure the output side if referenced to ground...but no inductance to cause fits of transient over voltage.
If you have a detail circuit in mind, post it, and we can walk through the reasons behind any guidance.
cheers,
Douglas