Mark,
I'm a B&O dealer, so I'll give you my perspective.
The way you've gone about this post, I assume that you're a pretty reasonable guy to deal with. When a client (even one who doesn't have past purchases to leverage) is reasonable and candid, negotiations are generally quick and smooth. I'm likely to tell someone exactly what I can and can't do. As the saying goes, you catch more flies with honey.
As for the raw numbers, there is very little profit in our industry. We have to manage jobs very carefully to be sure that our clients get the best service and we make enough money to remain in business. Depending on sales, a few percentage points could be the difference between profit and loss. Being open with your dealer is the best way to save where you can. The best savings come not from discounting, but altering the plan to make more sense. If 9s are more than adequate and you've requested 5s, than a smart product choice could save you $10,000. A discount may only be a few hundred.
Do not also think that labor is a piggy bank waiting to be cracked. An installer's time, whether in-house or subcontracted, is a fixed cost just like a speaker. A dealer who doesn't present the value of a good installation either can't provide one or won't be in business long. Our products are great, but what makes my stores competitive is how professionally we execute the installation. Pre-wire, termination, mounting, programming, testing, wire-dressing, and return visits for fine tuning are time and labor intensive and are crucial to a robust system.
The most honest appraisal I can give you is that there's no way to save more than a token amount without actually cutting products. There's simply not a profit margin like there is in furniture. Most dealers will give you a break, not for your loyalty, but for your attitude in dealing with them. Custom AV can be tricky and complex and clients who are easy to deal with get discounts, freebies, inside scoops, etc. Ones who are demanding and impatient are lucky to get a return phone call after the job is complete. Remember that it's not the top-line dollar amount that gives you leverage, it's your attitude combined with the profitability of your business. Don't over-value a small savings.
Best of luck. You can PM me if you'd like to go more in depth.
There is scarcely anything in this world that some man cannot make a little worse and sell a little more cheaply. The person who buys on price alone is this man's lawful prey. - John Ruskin