With wires, we worry about Amperes.
But let me assume it's 8 Ohms and 340 Watts RMS. I won't go deeply technical here but let's see if we will be pushing our luck with say 100 feet of 16 gauge copper.
100 feet is 0.40160 Ohms for 16 gauge copper. So we're fine if our 8 Ohm speakers lose a few Watts along the way. Purists will squeal but it will be fine. 14 gauge gets us a little less at 0.25250 Ohms which is negligible.
As the few Watts are spread out along the 100 feet of copper, heat is a non-issue.
There are insanely long discussions about Watts in receivers versus Watts of the speaker but it's rare to see folk push what looks to be 740 Watts of audio power in your den. If I was to do this I'd shop for the mid range of that power maximum in the amplifier. I would not try any 20 Watt amp.