I think the Harbeths are the ones you should listen to. I have not heard the SHL5s, but the C7s, M30s, and M40s all play superbly well at low volumes for dynamic speakers. The Quads are still king at that, and the 63s and 988s have that "of a piece" quality which no multi-driver speaker can match, but these Harbeths are not too far behind.
The Harbeths have equal or better bass, better and cleaner highs, and the midrange actually beats the Quads. What really swings it for me, however, is the fact that the Harbeths, while they won't play disco loud, play demanding material at naturally high levels and a bit above--levels which will either fry or shut down the Quads.
Robert E. Greene of TAS owned Quad 63s for more than 10 years before moving to Harbeth M40s as his reference. He has now used the M40s as his reference for more than five years. He likes the Harbeths better and finds them to be yet more accurate reproducers of music. I agree and now own M40s. Check out the Harbeth user group (HUG) archives.