I 'upgraded' from Gallo 3.0s to Harbeth Compact 7s a few years ago. I selected the Compact 7s over HL5s due to room size. In my opinion both Harbeths and Gallos are excellent speakers, and it's hard to go wrong with either.
They are very different, and an extended Harbeth audition is almost a must. The Gallos have more extended, but still smooth, treble; however, the Harbeths portray more inner detail. The Gallos have much better dynamics and bass. If that is what you're into, the Harbeths will not satisfy. On the ohter hand, the Harbeths have that magical midrange. I don't know of any speaker that does better with stings and vocals. You are correct in your assessment that the Gallos need to be played at a certain volume level for optimal performance. The Harbeths sound much better at lower volumes, but will not play as loud as the Gallos. Ultimately, the Harbeths are the only speakers that I've ever prefered to Quad ESL-63's in my own system.
It comes down to personal preference. You won't go wrong either way.