Since you predominantly listen to vocals and small ensemble, the better tube amps might do a better job than your current Naim. I do not have experience with Sugden but have read about their smooth and warm sounding signature.
Which Naim amp do you own? Nycjlee gave some good recommendations on solid-state amplification for the Harbeth. LFD, Naim and Exposure are more on dynamics (I have experience with LFD and Naim). As for the Hegel and Sugden, my impression is these are smoother and darker sounding amps, less dynamic and fireworks. Best is to listen and decide for yourself.
FWIW I chose Naim separates (NAC 282, Hicap DR and NAP 250 DR) to match with the Harbeth SHL5 Plus as the Naims are able to deliver the pace, rhythm and dynamic swings in the music very well with the Harbeth. Ditto the LFD and Exposure (the LFD has a slight tube-like glow despite being solid-state). If you want more sweetness or tube-like warmth or glow in vocals, there will be other amplifiers, especially tube amps that may be better than the Naims or any solid-state amp for that matter, though there might be a slight compromise in areas of speed and dynamics. The Harbeth may sound slow and uninspiring with certain solid-state amps such as Rega and Plinius.
As usual, the above are my experience based on my system and preferences. YMMV.