The most ideal circumstance for long interconnects is when the interface is balanced AND the output impedance of the component driving the cable is low. So I second Ron's question about what preamp is being used.
The higher the output impedance of the component driving a line-level interconnect, the more important it is to minimize length, although that importance will be greatest in the case of unbalanced interfaces.
Also, low cable capacitance will work in the direction of making longer interconnects acceptable.
Factors which increase the importance of keeping speaker cables short include:
1)Low speaker impedance.
2)Wide variation of speaker impedance as a function of frequency.
3)Damping factor and woofer control being particularly important to the particular speaker.
4)High output impedance of the amplifier, which is equivalent to low damping factor. That is typical of tube amps, consistent with Joe's comments above.
In general, my vote goes to long interconnects/short speaker cables, particularly in this case since the interconnects are balanced.
Regards,
-- Al