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

It all depends on what's in the "network," Bob.

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

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.

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.

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.

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

I've got the same question but I'm only extending from 9 feet to 14 ft. I bought some Signal Cable biwire speaker cables and they sounded fantastic. The detail in the low end was amazing. Now I need a longer run (14 ft) for cosmetic reasons. Will I be able to hear much difference in the extra 5 feet? I'm driving a set of B&W CDM-1s.

Thanks

Assuming you use 12ga cables you will lose about 1db in the cables. SPL loss will be less for larger guages and more for smaller guages. Besides the spl loss there is the obvious expenditure for such long cables. Now to answer your question about quality. There will be some loss in sound quality (perhaps a slight haze or loss of detail and esp. low-level detail) if your were to compare 8' to 40' cables. If it were me I would rearrange the system and furniture.