Signal degration over length, source vs. output ?

If you have a solid state preamp with output impedance in the 50-100 ohm range, interconnects are not all that critical. As Kr4 says, impedances for this interface (preamp to power amp) are well defined, whereas the power amp-to-speaker impedance relationship is variable. Therefore go with long interconnects. I note that at recording sessions it is not uncommon to have a hundred feet of interconnect wire between microphones the mic preamp and the recorder.

If you have a tube preamp, with the typical 600ohm output impedance, keep the interconnects short.

These general rules apply for "decent" (affordable)wire. The disadvantages of long wires of either type can be overcome (so they say) by various kinds of exotic wire configurations, for which you will pay dearly.

Kr4...I didn't know that you could get 50 ohms output impedance with a tube output stage (usually a 12AU7 cathode follower: 600ohms). What kind of preamp do you have, and did they slip in a transistor line driver?

sean...Yes: balanced lines are better for long runs, but that's another discussion. I only meant to suggest that a long run of low level signal is not the end of the world.

Kr4...Your SFI preamp looks like a nice unit. They confim that the output stage is "parallel tube with feedback" (not a transistorized line driver as I suspected) providing output impedance which they agree is low for a tube preamp. Looks like you get any tube advantages without one of the drawbacks.