Make sure you are well capitalized. I would not start such a thing without 2 years of operating expenses in the bank. If I could not make it self sustaining in 2 years, I would fold.
I would also think more in terms of not losing money, rather than making it. Profit if you can, of course. But I would want to minimize sunk costs like inventory.
I think personalized service is a good technique and sets one apart from the usual limited service providers that only work in-store. People do pay more for that and they are very loyal I found after they find the personalized service. More Moola!
I would say going almost all service makes sense. Here's what I mean, thinking about how I would do this thing I would never do.
Have a few systems set up, but use them for diagnosing tastes. A warm, neutral, and bright speaker set up, 3 amps to switch with the 3 speakers. Ideally you would want different price points, so maybe super cheap/$500 (so you can help any young people who wander in), then 3k and 10k. The specifics are not important. One of your services is "taste diagnosis", terrible name but it would help people avoid mistakes.
You can also sell that gear to walk ins.
Offer to construct build sheets to fit the taste and budget. This way you can get money from people who will buy online.
Offer to buy the build sheet, likely on a cost plus basis. Offer to install it.
Offer room measurement and system optimization services. (another way to get money from people who will buy online).
Ask what they have used, what music they listen to, room size, any limitations on design (no black, likes wood finishes, etc). This could be a start, a web form, a paper form to fill out, q and a in person.
Sell your knowledge and service.
Good luck!