I would advise you to check out the
AVS forum, and ask there regarding the options. You could get a
relatively cheap used 5.1 AVR that supports at least DD and DTS formats, possibly with a set of corresponding speaker set, that's ain't a bad starting point at all as for Home Theater audio.
But the limit is the sky both in quality and price, for HD 7.1 audio support you will need a much-much more pricey AVR and speakers.
Till, I would advice that you connect your current soundbar with the optical input to the Box and set the SPDIF to RAW in the Playback menu, this way you will have DD in 5.1 at least. So when playing a video (and there is such a track), always switch the audio track to DD (AC3) in the Player menu and you can still enjoy 5.1 surround.