Replace it.
I don't mean this in a callous or off-hand way. I looked around for more info on this and, quite frankly, it is what it is. If you are not happy with it, I'm afraid there is not much you can do.
The good news is that you may be able to replace individual components over time, but even this I'm not sure of. There is no Ohm rating published anywhere. My suspicion is that this is one of those odd HTIB packages with some strange Ohm rating, like 140wpc @ 3 Ohm. The receiver may not even be capable of driving a normal 8 ohm speaker system.
I hate to be the bearer of bad news, but I think you're in a love it or lose it situation.