Here's a reply I got from Billy at Cary Audio half a year ago when I asked him about the difference:
"The SLI-80HS is a versatile amp. You can switch from triode mode to UL mode on the fly. A lot of people cannot discern the difference in sound quality between the two. To me, triode does sound better but it is subtle. UL is more powerful. With the SLI-80HS, you have the ability to tube roll. You can use different tube types like the EL34, 6550, KT-88, KT-120, etc. You can also use different brands. A number of people enjoy doing this.
"The SLI-80HS is a versatile amp. You can switch from triode mode to UL mode on the fly. A lot of people cannot discern the difference in sound quality between the two. To me, triode does sound better but it is subtle. UL is more powerful. With the SLI-80HS, you have the ability to tube roll. You can use different tube types like the EL34, 6550, KT-88, KT-120, etc. You can also use different brands. A number of people enjoy doing this.
The SLI-100 is a more powerful amp than the SLI-80HS. It only uses the KT-150 tube.
Both amps are excellent. Sound quality and performance are outstanding. Just depends on what you want."