Large merchants have discount deals that change based on demand. If they are trying to move large volumes that did not sell in the previous quater, there will be some great deals. If their product is in high demand, and their inventory is low, there will be little, or no price break. So, the prices can change with little, or no warning. The on-line stores deal in volume. Most of your local shops buy in quantities of a dozen, or less. This amount can vary, depending on the product.
(i.e.- JL expects to sell 25000 subs in the first quarter, but only sells 15000, they will sell the remaining merchandise at a discount...to Woofers Etc for example... to recoup their invested cash.)
Then there is store cost overhead. They have to pay employees, utility bills, rent, taxes, etc.(local stereo shop) While on-line stores don't have nearly the amont of overhead, and order a truckload at a time, instead of a pallet at a time. Volume buying is always cheaper. This is a brief overview, other factors apply, and it's by no means a complete answer. But, it should give you a little better insight into how they set their prices.
I do purchasing for a corperation, right now. It's not car stereo stuff, but the principal is the same.