The only stock seat Triumph makes that's worth a ****, IMO, is the Thunderbird. It's about as stiff as a board. BUT, that's what you want on a long ride. Soft, cushy seats feel good in the showroom, but they'll have your back reeling on a longer ride.
p.s. I have a T100 that I bought about a month ago. I had the Thruxton seat put on it before taking delivery. I haven't taken it on a long ride, but I ride it for 100 miles or so about every week. I don't have any complaints with the seat thus far. Anyway, time will tell but I may have to add it to the "acceptable stock seat" category.