Are cables additive or subtractive?

Both it seems. I use them as small final sound adjustments. Sometimes it adds something I like and sometimes it takes something away I like. I have about 8 sets of different ic's. And to me I t seems I can count on them changing the sound the same way in every system. I kind of like that. But like all of us I have just limited experience. 

@marqmike - Can you be more specific?? Like cables X adds Y and cable A subtracs B?

My impression with all cables (including mains) that I have thought an improvement on what I was using previously is that more of the music is getting through to key components of the system, and ultimately to my ears; or to put it another way, the musical signal is being better conveyed. This implies subtraction: the reduction of hash or interference that otherwise impedes musical fidelity – but not necessarily subtraction from 'the signal'. 

Depending on your equipment and the room, cables will make a difference. If you have cheap junk gear, you will most likely not hear any difference. Types and brands all sound different. case in point. Same gear, same room. Speaker cable "A" I already own and happy with. sounds great. Cable  "B" brought over by dealer, $1300.00 cable. Sounds like crap, like wet blankets were put over the speakers. Cable "C" recommended by another dealer. delivered more focus and detail. $1300.00 as well. I own them now. Yes all cables will make a difference as long as you have decent gear top start with.