HH - string - Hub Host address ( DNS or IP )
What means a simple URL or IP no port given
If you look @ the example given alto it indicates also the port suffix to be included.
HHexample.org:555
So what is the real spec ?
The last seems logic besides 1 point , if this HH is going to be used to inform a hublist about a address change , why not send the complete qualifyed ADC address like:
HHadcs://example.org:555
(and maybe this field could then maybe 1 day be used to autoupdate the clients favorites

That way it could be realy used to change a adc hubs address to any ADC(s) compatible address.