distortion has many definitions, I wasn't too sure what you meant by "flatten (ie, distort)", I regard distortion as changing a wave shape e.g. something that starts out as a sine wave should emerge as a sine wave. But the purist could argue that changing the dynamic range is a distortion of the original, DSP will also add add digital noise, I think distortion that can only be measured by instrumentation (rather than the ear) is acceptable
Pitch shift would count as sine-->sine, too.
It is not my specialist field, but amplitude compression will always introduce distortion, won't it? You can hear that in outside broadcasts where there is background noise that switches on and off as the reporter speaks. (Compression is also used to mean information compression, to add to the confusion.)
Anyway, amplitude compressed music is easily told from the pure article, at least for instrumental music.
As for the OP's point, I think the problem lies with the broadcaster. It is up to them to decide at what volume to mix dialogue in, and often they choose too low a volume, as they want exciting crashing sound effects to jolt you out of your seat. That is OKish on a TV speaker, where most of the sound is lost, but on a better system it means that when the dialogue is audible against the background noise of the living room, the effects are sometimes really loud.
So, maybe the broadcast centre channel in 5.1 is too quiet? I don't see that as Humax's problem, unless they are fiddling with it, and I hear no evidence of that, as different sources all have similar problems. Compressing the whole 5.1 signal would just make it worse!