Having lived in and around NYC, haggling or dickering or bargaining is considered by some an art form. Myself, I don't worry too much about the long term relationship. I would want to get an idea of just what services I can expect at the store in the event that an item needed to be serviced. That might mitigate matters somewhat.
For starters, I'd hunt around for online prices to get a feel. S/H becomes a part of this. If I could find say 3 places, I'd strike an average. For arguments sake, I'm assuming this is just speakers. Let's say the discount you found was 18-22% from online shopping. Further let's say you're prepared to to strike an immediate deal at 22% but you'd like to do a bit better. Let's say 25%. Go to the dealer and offer something less. 28% off. The dealer will balk. Tell him your wife or girl friend will hit the roof if it's less. He'll still balk. Give him your business card and write your number on it. Tell him to give you a call if he can do better and walk out. If he calls, hear what he has to say. Still no good? Tell him you'll discuss it with your wife. Come back in a couple of days with her. It's time for good cop/bad cop. Show her the speakers...fawn over them. Let her say loud enough that the salesman hears it's too much. Approach the salesman with your wife. Explain how you can't get his price past her. He'll say something like 'well what price works for you Mrs. Beebe? She'll come back with something close to 25% off. He'll probably counter with something awfully close. Then buy it.
I've heard this relationship thing for years. Develop a relationship with your car dealer, with your clothing store, with your jeweler, etc. I'm reminded of a quote made by some British statesman..."We have no eternal friends. We have no eternal enemies. What we have are eternal interests." It's in your best financial interest to strike the best deal possible. The dealer won't sell for less. In the end he's going to try and hit you up for cables and a power conditioner. Like taking your kids through the candy aisle at the supermarket.
Dont be rude about it. But just start low and work up. You never know they may go for 20% off.