As others have rightfully pointed out now — it’s simply a popular amp that’s been out for several years. If you’re looking for nefarious undercurrents you’re not going to find them here. There are simply a lot of units out in the field, and over time, many of those owners will decide to upgrade or leave the hobby for various reasons. Add to that many now getting spooked by the current tube shortage — this specifically affects the best KT120 and KT88 options for these amps. I have monoblocks that use 16 KT88 and another set of monos that uses 8 KT88, and then yet another which uses 12 KT120 (Rogue Apollo Dark) and I’m not budging from tubes but everyone’s different lol.
Anyways, if you were interested in the ST100 it’s a good time to pick one up or bargain with the sellers. Great amp! For even cheaper you can look for its earlier iteration the Stereo 90. Also a great amp.