Hi John,
I’ve been a stalker here for a few months, and finally registered tonight. I saw you asking for feedback on promotions, and thought I’d throw in my two cents. However, please take both pennies with a grain of salt, seeing as finance is my forte, and not marketing. 
Of all the promotions I’ve seen from the inside, the ones that were the most successful (both short-term impact and long-term satisfaction) were the ones that improved the product we were selling. That’s why I think the free SPL meter is genius. Here are a couple of other potential examples.
Let’s say that from your experience, customers buy great audio systems but skimp when it comes to speaker wires (I’m not sure if this is true or not, but this is for illustrative purposes). Let’s also assume that Monster XP wire would be a huge improvement to the overall system’s performance, but the XPHP is noticeably better than that (again, for example only since I’m not sure of the quality gap between the two wires). Then a promotion such as “A free XP wire kit or 80% off of a XPHP wire kit” would be ideal. It will make your product appear even better, give many customers something they would have needed to purchase anyway, and act as an incentive to upgrade further to the XPHP.
As you said, the same promotion all the time ceases to be a promotion. Another example could be speaker upgrades. Let’s say that you feel your 12 inch subwoofer is much better than your 10 inch, yet your 10 inch is the most popular. Then you could run a promotion such as “buy a 5.1 system with a 10 inch sub and get a free upgrade to a 12 inch.” Again, the customer gets more for their money, and Aperion benefits by improving the overall quality of the product in the hands of their customers.
I’ll probably be putting in an order around 2/1 when you get your shipment in. Maybe I’ll get to take advantage of a new promotion then. 
-Dan