I ran a pair of 7.5 meter, unbalanced interconnects from preamp to amps, with very short speaker cables, for years with no problems. Recently, I replaced my 30-year-old interconnects with Morrow Audio cables, which are unshielded. This caused some noise problems--my amps hummed more than usual and turning on or off a lamp made noise in the adjacent speaker. So Morrow will be sending me a specially-made shielded version of the 8-meter interconnects, and hopefully that will solve the problem while still retaining the superb transparency of those cables.
With shielded interconnects, running unbalanced is unlikely to be a problem in an ordinary domestic setup. In a professional setup where hundreds of feet of cable may be required, balanced cables are appropriate. If ordinary mic cables can be used for a balanced setup, that could save $$, as long as both amp and preamp have the balanced jacks. Balanced cables can transmit a higher output, therefore lower perceived noise level than long runs of unbalanced cable.