olddog,
It seems we're having trouble answering your question... "What $300 sound bar?". I also don't have any experience with the long narrow ones you want for the mantle. I do have a couple of the square, flat sound bars that go under the TV. Mine are both Denon, and were $200. They have 2 cords... a power cord and an optical cable to the TV.
They are a great improvement over the TV speakers. They have mono, stereo and surround modes, but I can't tell much difference between them. They also have blutooth so I can play music from my phone wirelessly. They are auto-on, auto-off, "learn" your TV remote so you don't have to use another remote for the sound bar. In short, we like them and they're worth every penny of $200.
I know you don't have room for a square unit, but in your situation and knowing what I know about the Denon square sound bar, I would be willing to try one of their long skinny ones. Sorry I couldn't be more help.