Laura, if even a floor lamp is contributing significantly to this buzz/hum, it would seem to indicate a shielding problem in the amplifier. An inaudibly low noise floor(which I assume is synonymous with "blacker background")should be routine with any competently designed modern receiver or separate amplifier, regardless of cost. The least expensive of my receivers(factory refurb, $151 delivered)has such an inaudibly low noise level and overall superb performance as an amplifier. It's unlikely that different amplification is something that will improve your overall sound quality, as that is determined almost entirely by the recorded material, as affected by your speakers and listening room.
