From what I can tell, that is a pre-amp, not an amplifier.
http://www.marantz.com/p_product.cfm?id=399&cont=u&line=amp&cat=hf
The very little bit of sound you are getting is from line level output, it doesn't actually have an amplifier in it. All it does is process signals and it is designed to send those signals to an amplifier. It is, however, a very nice pre/pro and if paired with a solid amplifier should give you many years of great service. Hope this helps a bit.
Mike
PS - It looks like the MM9000 would be the amplifier designed to match with this unit.