Hi Mik -
Happy Holidays. One of their amps that I own is the multichannel 38. It leans cool. I find this exceptional on the bottom end. I have planar and ribbons on my speakers on the top end. I've heard several amps with them including the 38, ST-9 and another amp under development from another company. I also have a dealer that loves tubes.
From my experience I LOVE the 38 on the bottom in 400 watt config. The dynamics, detail and control are exceptional. This is where it gets weird. With a speaker with reference upper end that is very neutral - you can REALLY hear the difference between the amps. While the 38 is great, it can border on analytical to me. The unnamed amp was very rich and had a lower bloom to it - not really accurate, although quite fun. I'm a fan of the ST-9. It's accurate to the warm side of neutral. You can refer to the NuPrime threat. Jason explained the circuitry on his thread which has a long conversation about the circuits between his amps.
I think the best thing you can do is A/B with the ST-10. You should be able to hear the difference right away. Mind you I had developed two final crossovers for my speaker - both sounded great with subtle differences. One was somewhat luscious while the other was deadly accurate. In the end I chose the latter.
Best I can say is try to have fun and don't pull your hair out