If your line magnetic 518 is this: https://www.stereophile.com/content/line-magnetic-audio-lm-518ia-integrated-amplifier-specifications
Than you should not be worried about the cablelength...just keep it with low C. The nature of Tube Amps is of low to very low DF. As so, cable's resistance is insignificant.
Than you should not be worried about the cablelength...just keep it with low C. The nature of Tube Amps is of low to very low DF. As so, cable's resistance is insignificant.