If your Cornwalls are a flat 8 ohm load then try resistors on the speaker terminals to get it down to 4 ohms. The SIT-3 sounds best at around 4 ohm loads.
I don't remember which size resistors. Zu Audio would know and can make you a pair for your speakers.
I have Frankenstein Mk.II 300b mono's and a SIT-3 amp which I use with my Horning Eufrodite Ellipse speakers. I do like the 300b's better, but for SS amps they are really nice!