I have a pair of QS Mid-Monos, the latest version with the 2 bias pots, with EL34, that I regularly rotate in my system. Currently they drive a pair of Harbeth 40.3 XD. I use new Tung-Sol tubes with which I replaced the tubes that they came with the units. The other tubes are those who came with the amps. I bought the Mid-Monos used.
The QS have no problems driving the Harbeth in my medium sized room.
Here’s how I would describe the sound with the Harbeth in my room.
There is plenty of resolution, tonality, and warmth. Not much punch in terms of dynamics but an addictive and intoxicating mid-range. Bass is fine but a little loose. Overall, the Mid-Monos are excellent amps, a great deal, even a greater deal if found used, I think I paid around $1300 for mine, made in USA, and Mike Sanders is an email or a phono call away to answer your questions. I got a response to a question that I had in an email in less than a day.
There is nothing better imo to be able to communicate and get support from the designer himself.