Hard time finding comfortable, affordable sofa

So far I’ve looked at FlexSteel, England, Lancer, and Bassett brands as they are in my price range (most around or below the 1.5k mark). My issue seems to be that the frames aren’t firm enough. The options for firmer cushions haven’t felt much more comfortable to me. We’ve got an old FlexSteel that sits just fine, with a very firm base that travels evenly and lets the cushion take the load. The new stuff I’ve sat on doesn’t come close. So many of these new sofas I sit on sink in the back instead of moving evenly, and the cushions fold in the center and pull apart from each other. Where else should I look?

Unfortunately I don't know of any well-made sofa in that price range, the market has moved beyond that price tag for even mid-quality. At $ 1,500 you are really going to get not much more than a stapled frame with webbing suspension and economy foam with a polyester cover. If that's your absolute budget, check out pre-owned, better quality furniture in your area. People put high-end sofas for sale all the time due to style preferences, or moving and that's a good way to go.

What should I expect to spend for a mid-tier piece, and in which brands?

I think you should plan on a $ 2,500 budget to get into mid-tier builds on domestic fabric sofas. Stay away from the Mall and Catalog furniture stores, they have the highest markups. Shop an independent. All the information that tells you what a quality build would be is right here on the forum.