ETA: Systlin just posted to say that @thebibliosphere has hooked her up with a beta and a potential publisher. I'm going to leave the post up so others can possibly use this knowledge but please refrain from further publisher suggestions and selfpub advice!
Unfortunately, if what you want is to maximize both sale volume and revenue -- which is a very normal goal, I'm not taking a swipe at that or anything -- Amazon is probably the way to go. I write for fun and have a large readership already so I didn't necessarily need Amazon, and when I was on Amazon it forced me to price my books higher than I was comfortable with, which is why I backed out.
As an alternative to Draft2Digital, I use Lulu.com. The mechanism -- how you upload your book and what information you include -- is functionally the same, but Lulu doesn't charge a setup fee, allows you to edit your text multiple times without fees, and is slightly less expensive per-page. So if you don't want to deal directly with Amazon, Lulu is superior to Draft2Digital in my opinion.
Lulu will allow you to sell through other platforms such as Amazon, Barnes & Noble, etc, but it's opt-in, it won't do it automatically, and if you do that it makes you price the books so that those platforms aren't taking a "loss" by their standards. So a book I publish on Lulu might cost $10 to print (that's Lulu's cut) and I can sell it for whatever I like above that on Lulu -- say I sell it for $20, I get $10 per book. But the same book would "cost" Amazon $21 since Amazon takes a much larger cut, and Lulu's retail price dictates Amazon's. So in order for Amazon to sell my book, I have to price the book above $21. If I price it at $22 and someone buys it from Lulu, I get $12, whereas if someone buys it on Amazon, I get $1. Lulu will never be more expensive than Amazon but Lulu will always take Amazon's cost into account if you hook them up together. (I just pulled numbers out of the air for that example, but for a real-life one, when I took one of my recent 6x9, 90K-word paperbacks off Amazon, I was able to drop the price from something like $18 to $12.)
Selling directly through Lulu without involving Amazon is thus absolutely the most profitable way to go except that you don't get Amazon's reach or universality. Buying from Lulu, people have to know it exists, navigate the site, and trust that it's legitimate. Buying from Amazon, someone might just...find your book, and people will go to Amazon just to browse, which they will not do on Lulu, especially since as a self-publishing site, Lulu is full of garbage bad books. So if you're publishing outside of Amazon you will have to do a lot more publicity and you will never have the same sales volume.
I will also say that when Lulu published my work to Amazon, something got truly fucked up -- I'm not sure who did it, but either Lulu or Amazon assigned someone else's book to my author name and fixing it was really hard because I wasn't the one who set it up, Lulu was. I couldn't just go in and remove the book, I had to talk to Amazon and then Lulu and then have Amazon and Lulu talk to each other. I think my author page on Amazon might still list a book by Kathleen Starbuck as one of mine. There's also some weird shit around adult content -- I believe to sell on Amazon through Lulu, you have to jump through some hoops if there's adult content by Amazon's standards, which I know includes erotica but may also extend to queer/poly non-erotic content.
So yeah, I think depending on your goals, Lulu is worth exploring, but if you want to make life easy on yourself probably the best thing to do is just hold your nose and go with Amazon. However, if you're interested in Lulu I'm happy to answer questions.
Also, if you're interested in publishing in such a way that you can have your book sold in stores and held in libraries it will need an ISBN for the print version and a separate ISBN for the ebook. If you're in the US those cost $125 each unless you buy in bulk, which I happen to have done, so I have like 80 of them. If you want a few I'm happy to give you some for the at-cost of $5 per, but there's no way to transfer ownership of the number itself, so you'd officially be published by Sam Starbuck / Extribulum Press (you'd retain rights/profits, I'd just be your publisher on paper). I know Lulu will also give you one free and I imagine Amazon would, but often that means they retain certain rights to the text that I've never been comfortable letting a corporation have.