I think you want the best cables on the front end (ICs) where the voltages are much lower and RF interference or intermods are subject to amplification.
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I think I understand what you are trying to accomplish. You are afraid that the lower quality Nordost Blue Heaven i/c between your preamp and amp may be the limiting factor if you consider higher quality(more expensive) speaker cables and i/c between your source and preamp.
I guess you will never get a general consensus on this. Since I am not familiar with Tara Labs i/c's, I cannot guarantee whether you would be able to obtain an improvement if you were to upgrade to the Nordost Blue Heavens. However, there is no wrong in using a higher quality speaker cable and a higher quality i/c between your source-preamp together with a lower quality i/c between your preamp-amp. The only issue that may be of interest to you is whether the improvements you obtain would be in line with the amount of money you have spent on the cables. That only you can answer for yourself.