First of all, I agree that you should always ask about discounts. Nobody should be offended by your questions, and if they are, then simply look elsewhere. I've found that most high end audio dealers are pretty straight forward about what they can do for you price-wise. It depends on how much you are buying, how much flexibility they have with the particular company's products you're interested in buying, and of course how serious you are as a buyer.
If they are dealers that you've bumped into from an online conneciton (Audiogon or otherwise), always call and talk to someone early in your decision making process. I've done this many times, and it always helps to break the ice. Talk about the options you are considering, what your system now consists of, and what kind of sound you're looking for, but always express a direct interest in purchasing a particular product that they offer. Dealers tend to be very friendly but they don't like tire kickers as a general rule. Have an objective in mind when you call them, and don't pressure them about the "pennies" (i.e., the exact deal they can offer you) until you're ready to make a purchase.
Just try to get a general feel for what the dealer can do for you and focus more on building a good rapport with the salesperson. Then when it comes time to deal, maybe months later, they will be more inclined to do the best they can for you because they remember talking with you before.
If, however, you know EXACTLY what you want and are only price shopping, then do them the courtesy of being direct, open and honest about it. In other words, "I've done a lot of research, and I've been auditioning speakers all over the place for months, and my heart is set on the Maggie 20.1's (lucky you). I'm 100% ready to buy a pair, and it's just a matter of what kind of a deal I can get." If you haven't wasted 20 hours of their time already, I'm sure this would be refreshing to hear, and they can react accordingly. More often than not, they will tell you exactly what they (or any other Maggie dealer) can do. But this is only if they know you are serious and ready to go with the deal.
As far as the discount itself, it depends (mostly) on the policies of the companies whose products you are intersted in purchasing. I've never purchased any Krell, AR or Maggie gear, but I suspect the 10%, 15%, and 20% types of guesses offered above would be about right. There are other high end companies that offer much higher discounts, sometimes 30% or even 40% on speakers and amps, and 50% or more on cables. But certainly not Krell, AR, or Magnepan.
These are all well established companies and they all have well established niche markets. For example, a lot of folks are exceedingly brand loyal to Krell. Why not? Krell has always made excellent products, they have been around for a long time in a very tough industry, and they can afford to advertise extensively. But Krell seems to attract a LOT of 'up and coming' types of audiophiles. This is especially true of high income professionals who are trying to put together a killer HT system and then be done with it for the next 10 years. They types of people have yet to discover Audiogon, and might not even be interested in becoming a "hobbyist" even if they did. Many of them have decided that they want the "best" (whatever it costs) and Krell has a great reputation. Yet, Krell gear doesn't necessarily represent the best price/performance ratio available today, or at least not for their entire product line.
I'm not suggesting that ALL of Krell's customers (or AR and Magnepan customers that matter) are "semi informed" and are not interested in researching other alternatives any further. But I do think there is a high enough percentage of these types of customers that Krell sees itself as being in a strong position in the market. Thus, they don't give their dealers much price flexibility, knowing full well that they will lose a sale occasionally as a result.
But there are not very many high end audio manufacturers who find themselves in such a strong position as Krell or AR. In other words, there is a LOT of competition in every niche in the market for speakers and amps. The industry is spread very thin, so the biggest problem that most high end audio manufacturers have is in sustaining volume. No matter how great their designs are, they need to make sales to survive! Everyone wants into the game and there simply are not enough customers to go around.
To combat the heavy discounting that some dealers may wish to engage in, a nummber of manufacturers have established wide dealer networks (sometimes hundreds of dealers across the country), and thus maintain tight controls over dealer pricing and will kick out any dealers who have violated their agreement. Other, smaller companies, may use only have a handful of dealers (sometimes as few as 3 or 4, but more often 10-15 dealers). All of this affects what kind of deal you can get, and helps to explain why it tends to be very company specific.
Depending on where you live, some companies may be able to sell directly to you and at a better discount than their dealers can offer, but they have to be careful not to step on dealer toes. Living overseas, I've benefitted from this knowledge on many occasions. More often than not, I can deal directly with the manufacturer or their US distributor, so long as the trasaction is set up in such a way that they ship directly to me (overseas) or to my freight forwarder. Because I live outside any of the dealer territories, this cuts out the middle man's share of the deal. They can then do a little better for me and not be any worse off (or even better off than if I had gone through a dealer) themselves. I suspect that some of these same opportunities may be available to someone who lives within the US or Canada, but in a fairly remote area (I'm just guessing, and not basing this on personal experience).
What it comes down to is that there are a lot of factors, so "it depends" is the best answer. But there is typically no need to pay full price.
