I had a very early production S-30 and it was definitely load sensitive. It sounded great with my Merlin VSMs, but absolutely wilted when driving my Verity P/Es. I understand (from a post in this forum by Ralph, IIRC) that Atmasphere subsequently modified the design and that later production was more robust. So, I'd be careful re the S-30 if you're looking at the used market.
Marty
Marty