I have to agree with bassdude. I have owned the 40’s and currently the SHL5+ and tried Quicksilver V4’s, Quicksilver Mono 120’s, and had a long term loaner Audio Research Reference 75 SE Stereo Amplifier and none of them worked well with the Harbeth’s. I owned QS amps for years and they worked great with my Vandersteen's and I still love them.
My Harbeth’s sounded so much better with a Musical Fidelity Tri-Vista integrated amp, with the 40’s, and with the Aesthetix Mimas and the AX7e on the SHL5+. I just don’t think Harbeth’s and tubes mix. To me it sounds like someone threw a blanket over the speakers with any of the tube amps I tried. But this is my personal opinion and YMMV. We all have different listening preferences and only our own ears can tell us what we like. Also, Harbeth uses Quad SS amps to voice their speakers.
My Harbeth’s sounded so much better with a Musical Fidelity Tri-Vista integrated amp, with the 40’s, and with the Aesthetix Mimas and the AX7e on the SHL5+. I just don’t think Harbeth’s and tubes mix. To me it sounds like someone threw a blanket over the speakers with any of the tube amps I tried. But this is my personal opinion and YMMV. We all have different listening preferences and only our own ears can tell us what we like. Also, Harbeth uses Quad SS amps to voice their speakers.