You wouldn't receive any benefit from hooking up an existing amplifier or receiver to the soundbar. The soundbar will only accept line level inputs and use its internal amplifiers to drive the speakers. And actually, the internal amplifiers, if designed correctly, should work better with the internal speakers than a stand alone amplifier to stand alone speakers. If designed correctly, the internal amplifier should be matched to the various impedences of the speaker as well as trimmed for proper bass and treble response. This is something that can only be done from the beginning design stage. In practice, this rarely happens in a reasonably priced speaker which is why for purest fidelity, seperate amps and speakers almost always come out on top. But you don't get really good sound from those either unless you are willing to do the proper research and spend the proper money.
If anyone is old enought to remember, Harold Beveridge who was an aerospace engineer, designed some of the best sounding loudspeakers in history back in the 1970's. About 6-1/2' tall in beautiful wood enclosures. The drivers were electrostatic panels for frequencies above 100hz with dedicated subwoofers that were very clean to below 25hz in each panel. He built his own custom amps to match the impedence and capicitance of the electrostatic panels and to properly control the subs for even response down to the low frequency cut-off. The electrostatic panels were completely enclosed inside the wooden enclosure exiting from a 1" or so slit in the front of the cabinets. When you heard these, you didn't hear electronics or speakers, all you heard was music. With maybe one or two other very nice ribbon speakers, probably the most musical I ever heard. These were around $7,000 per pair in 1977 which means they would probably be somewhere between 30k and 35k today.
There I go rambling like Abraham Simpson again.