Many on this forum will tell you specs are useless. I am not one of those. Specs are, however, at best ADVISORY and can be misleading.
The human ear is very good at picking up distortion. BUT, as it turns out this ability varies as to frequency. At low frequencies, the ear is only poor to OK and 3% or 4% is nothing. Maybe even higher. But that same 3% at frequencies used by a Violin or Viola or higher frequency of the WoodWinds would drive you out of the room.
Other components contribute to distortion as well. The way it is measured also makes a difference. Than there are different KINDS of distortion. Harmonic and Intermodulation are just 2.
No longer simple, eh? Add in the fact that your room is doubtless less than perfect, the everyday noise of home life, any junk coming down the power line all make a difference and there you go.
So, as far as a single distortion number meaning much, I don't think so. As for your HK running 4 ohm, why not? If the speakers are not a wildly bad load the HK has a good reputation. Others with specific knowledge of this model will have better information. Are you planning a change of speakers?
0.07% is a good distortion figure. But it doesn't mean much. 0.05 is lower but that doesn't mean the amplifier is better. Total Harmonic Distortion is a specification that leaves out all the important information in order to provide a simple number you can look at. What is more important in judging an amplifier, if you are looking at simple specs is I.M. distortion and power bandwidth. Additionally, with harmonic distortion the components of the distortion are vastly more important that the overall number. How much second order harmonic does the amplifier produce, third, fourth and so on. Knowing that would give you a better idea of how the amplifier "sounds" though hearing anything much beneath 3% T.H.D. is probably about all you can expect. How the amplifier achieves its distortion spec is also important. You can get incredibly low T.H.D. specs on paper for poorly designed amplifiers when you increase the negative feedback in the amplifier circuit. Unfortunately, increasing the N.F.B. makes the amplifier more unstable into real world loudspeaker loads, so this T.H.D. thing ain't that simple.
Don't worry about specs, they do not reflect how the amplifier actually operates.