I have always found the Super HL5 Plus to have just a bit of a hard edge in the treble with some music. The midrange is of course amazing and they are very detailed without being overly "hi-fi" sounding. The Stirlings take that edge off while maintaining the beautiful midrange and are much easier to listen to with all types of music. I think they are a little more punchy and dynamic than the Harbeths. Where the Harbeths win is they are a little more coherent. Both speakers image really well and have a large well defined soundstage. These two speakers are more alike than different, the differences are nuances. If you want to rock out, the Stirlings are going to be easier on the ears. If you like to listen at moderate levels and love detail, then the Harbeths are a better choice.
Compared to the Reference 1s, the Stirlings have more midrange and top-end energy, but are perhaps a little less balanced and detailed. The Ref 1's are more neutral, a little on the "warm" side. Both speakers are really easy to listen to and the differences between them are more pronounced.
If you like the BBC "sound", the Stirlings are fine speakers. All comments above are in my opinion of course. Others may hear them differently. There are a number of reviews on the Stirlings on the web.