I owned Harbeth HLP3 and LS5/12a for 6 years and they are nice improvements on the LS3/5a. However, with small "woofers" you always get small sound. The 5/12's did have better bass than the P3's. Having heard the ProAc's I'd have to say that the Harbeth's have better music.
Another option, though I have not heard them, is the small speaker from Coincident Speakers (www.coincidentspeaker.com)
It's a little larger, and also has a larger (6 1/2") woofer. It's quite efficient (something that the Harbeth's are not). With the increased efficiency and larger driver, bigger sound and more bass should happen.
There's also the Audio Physic Step, but I've not heard those either.
My experience with the Harbeth's was that the lack of bass got tiring and I had to get a small, fast subwoofer (hard to get one that had a fast enough driver).
Harbeth eventually came out with a passive subwoofer for the HLP3, the X-Tender. The X-Tender also functioned as a stand for the P3's. The driver is same as in P3. I used the X-Tender with the LS5/12a's and the sound was great for most all music with a good bottom to the sound. I don't know if Harbeth still makes the X-Tender or not.
BTW, I no longer have any Harbeth speakers as they were sold last year when I got new, and larger, speakers.