The sound bar has its own limits, of course. However, the signals flowing through it aren't curtailed in any way. The sub should be getting exactly the same PCM / DD / DD+ / Dolby True-HD signal as it would if it were passing through a £5,000 AV amp. Your sub (when you get one) will do just fine.
If you want to test the limits of how low the sound bars subs will play, go to YouTube on the TV's smart apps. Search for Test Tones. These are a number of sweeps (rising or falling tones between two frequencies) that you'll find useful. There's one from 25-150Hz which is the right range. Grab your smart phone, download a sound meter app, use that to monitor the volume as the tone increases. You might have to fiddle around with the bar's volume to get enough level to make the meter reading react strongly.
Play the tone a few times to get an idea where there's a fairly quick increase in loudness. This will help you narrow down to the sort of crossover frequency you need for blending the sub.