Performance data insufficient

Preliminary, packaging and delivery were 1a. Everything is fast and good. Now the device. The envisaged 2x 10 Watts RMS into 8 ohms will definitely not reach (or a distortion, of listening to music makes it impossible). I have been controlling the CD player via mixer at 0 db, the sound controller were all on linear. The connected 8 ohm speakers began to oversteer at 2/3 volume that we have achieved volume was insufficient. As Hörreferenz I've used a Medion stereo (2x 5 watts RMS into 4 ohms) and a HiFi amplifier KD-261 (2x 110 watts RMS into 8 ohms). The KD is of course completely oversized, but shows what the boxes can. The Medion-conditioning controls, the same boxing much louder, even at maximum volume (CD music) not rattling the combination, in the living room this volume you hold out not long. The RMS power in an audible distortion is probably less than 5 watts into 8 ohms. Oh, without music it hums slightly and also the polarity of the speaker terminals was misleading (mass is at a terminal the color red !!). I send back the amplifier and buy a more expensive number.