Hard question to answer without knowing what kind of music you listen to and what your expectations are. I've owned the SHL5s, but now I'm using P3s in the same room and I think they do better for low volume playing. But, IMO, if your amp sounds good with your speakers at higher volumes, I'm not sure a more powerful amp is the answer for lower volumes. A loudness control would be best, but alas.
Recommendations for speakers that sound great at lower volume levels.
I have a pair of Harbeth SHL5 Plus and they sound wonderful when I crank them up. But at moderate to low volume levels they sound disappointingly flat and unengaging - instruments are less palpable, bass has less bloom, and soundstage has less air and dimensionality. I drive my speakers with a tube integrated - a Line Magnetic 845 rated at 26 watts of power. My Harbeths are rated at 86db. Would a higher sensitivity speaker be helpful? Or how about a good quality small shoebox sized pair of speakers coupled with a subwoofer? Or not. What speakers are going to deliver music you can feel at low volume levels? What say all you wisened audiophiles?