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Either your reception is being slightly 'jammed' by some other electronic activity in the region... and to check on that, just unplug or turn Off every other electric appliance around, and try again... or else, your BeoSystem radio's tuner circuitry is defective, and isn't centering on the center of the channel's band.
I think that last is fairly sure, because even with signal strength greater, owing to the antenna, you still have problems just as before... hinting that the signal was enough even without the added antenna, but it's defectively tuned in or defective in demodulation of the signal, somehow.
As for volume controls... try that elimination of interference/noise by unplugging or turning off everything else, first. If that doesn't fix it, tell me how you are trying to adjust the remote speakers' volume, etc. ?