You definitely want the TT to Phonostage short. Secondly it is good to keep the speaker cables short. A very common solution is to place the amps close to the speakers and move the rest of the equipment to the side to keep it from interfering with the speakers imaging. 15’ is not to far for single ended interconnects. But if it allows you to get a better preamp…we’ll, better is always better.
What does your venue look like? Speakers? If you put photos of what you have so far under your UserID it would be helpful in allowing us to envision your space.