40' speaker cable..will I loose sound quality?

At these lengths even the lowest impedence cables will show a real loss of sound quality. Try to avoid this setup if you can.

Those who are forced by necessity to run longer length (20'+) cabling may experience superior results by using a networked cable design - re: MIT or Transparent.
I had to do this myself & found that a better-grade MIT speaker cable performs marvelously in this application.

Warehouse: Unless you're using very thin-gauge cables, an extra 5 feet is not going to make a substantial difference.

Fcmcargo: What you have to worry about here is high-frequency roll-off due to high resistance. I'd try some 10 AWG cables for a run that long. And I'd stay away from networked cables--they're as liable to exacerbate the problem as anything else.

And I'd stay away from networked cables--they're as liable to exacerbate the problem as anything else

That is an absolutely incorrect statement.

I agree with Bob_Bundas on the networked cables. Also the larger gauge statement is correct.