More than size matters for great sound. It depends on the electronics of the amplifier, and the quality of the speakers and their enclosures. You need a wide band amplifier to get the full range of sound from your media (FM, CD, or DVD). You also need a quality source. Mp3 recordings sound good, but will never be excellent. AM radio doesn’t sound too good because of the limitations on bandwidth at the transmitter and received. The same problem exists with FM but to a lessor degree and also a better signal to noise ratio.
My guess is that the Panasonic was good for its time and price range. True audiophiles prefer vacuum tube amplifiers because they have a wider bandwidth and higher power capability. It doesn’t matter to me because my hearing isn’t good enough to hear the difference.
The only way to pick out a sound system is to hear it first. Much of the quality depends on the listener’s taste and ability to differentiate sounds. You obviously have outgrown the Panasonic. I suggest that you upgrade to a better quality system. Don’t waste money on speakers that are driven by inferior electronics.
Hope this helps.