IMO, never bank on a speaker "breaking in" if you think you heard an aspect you didn't like.
That said...
I owned the Harbeth SuperHL5+ for a while, and have heard it numerous times elsewhere, e.g. stores.
Given it's dimensions and rated specs, I did not find it bass shy - it sounded very full throughout it's entire range (given reasonable expectations). Of course if you place it nearer, say, the back wall you will get more bass re-enforcement. But even well pulled away from the wall I found it sounded quite full and I wasn't thinking "this is really lacking bass."
They are fabulous speakers. I sold them only because I felt I already owned speakers that did everything a bit better. But whenever I heard the Harbeths, as I just did again last week in a store, I fall in love with that sound again.