Again, speaker stands or mounting brackets that match the specific speaker well would be a huge plus. I use stands for my surround speakers, and I had to do quite a bit of searching before finding some that were strong and tall enough and looked good. Already having such items to bundle with a speaker would be added incentive to go with your speakers. Speaker wire, A/V interconnects, banana plugs/spades, speaker spikes... could all be an option in case one doesn't need anything else, but the matching stands I think would attract more attention from selling bookshelves, especially with higher end bookshelve speakers with nice finishes because who wants to hide away such nice wood finishes by sticking the speakers tucked away back on a shelf.
Combo pricing. If you buy main LCR speakers, get a discount on surrounds or a sub.
Your idea of getting a good sub eq at discount with a sub purchase is a great idea and one that would get attention of some customers looking for subs such as velodyne has with their eq'd subs. It's an acoustic treatment nitch that needs to be filled.
With any speaker purchase over X amount of $$, you get a discount coupon good towards any future purchases within say 6 months of your initial speaker purchase. Once upgrade fever hits, it's just a matter of facilitation to keep upgraditis rolling with bonuses/discounts to make it easier to keep shopping. Also, if someone is very happy with their purchase from someone and feels they got a good deal and well taken care of by the company, it sure makes it easier as a customer to go back to that same store/company for more items, with a good discount coupon or something similiar, like your AV123 family pricing, the customer is rewarded for buying more of your products.
SPL meter and set up DVD/CD has already been mentioned too, but it's such a good idea, it can be repeated.
At AV123, you already have such a great accessories/furniture already, you could simply give a certain $$$ amount off any accessory with certain $$$ amount of initial purchase.
Replying to this thread late in the game, I've repeated a lot of other comments, hopefully I've added some thoughts or expounded on something already mentioned.
Brad