Interconnects - better before preamp or after preamp?

If you drive a Ferrari do you put paraffin in the petrol tank? After all it's the world's best engine.

First is always the best. If it’s not preserved upstream it’s gone.

It’s all in the same chair = quality throughout.

As a temporary situation put the better cable after the source. But later on you should also get the best possible between preamp and amps. 

Your preamp does not improve the signal. The better the preamp the less it degrades the signal.  Consider that your source is a 100% signal. But the DAC/streamer/phono takes 2% of the goodness.  You're down to 98%.   Interconnects take 2%.  World class preamp only takes 1%.  next interconnects take 1%.  Amp takes 5%.  speaker calbes take 1%.  You get to the speakers with 88% of the quality that was on the source.   

Those thinking cables cords and components add something have it backwards. The artist added everything you're gonna get to the recording.  Your goal is to get as much of it to the speakers as possible.  Speakers job is to get it to your ears.

Jerry