Another cable length question....

I looked over all the previous posts and it is obvious that the answer is the shorter the speaker cable, the better. However, what is the longest length I can get before the sound deteriorates too much? Currently, I use 15 ft Signal Cables and they do just fine. However, my system has been moved to another room and although the speakers are not that far away, the connection between speaker and cable is too far for the 15 foot cables. I actually have to run one cable over a door way to keep people from stepping on it. This leads me to need a cable slightly longer that 15 foot; otherwise, the 15 footer is too taught as it is. The new placement of speakers will work for me but the cable length is causing me concern on good sound.

Can anyone help?