Yes they will. Even a better-than-cheap extension cable will cause some deterioration.
However, depending on lots of factors, you may not notice the difference at all, or the difference may be too slight to spoil your listening enjoyment.
If you're aiming for ultimate SQ, then your best bet is probably to completely re-cable with a long length of audiophile cable (that may end up costing more than your phones). But some say this whole area is just snake oil and it's certainly not the first step to take - unless you've already got your ultimate headphones. If you're not sure about the audiophile cable route, then give it a miss until you are sure. After all, by definition, it won't cost much to try the cheap one first.
Personally, I find a long cable for headphones is so, erm, liberating, that I'm prepared to accept some loss in SQ and I just don't worry about it.
)
