cary audio sli 80 signature vs willsenton r800i

Possibly.  What would the Willsenton accomplish that the Cary can't?  I'd keep the Cary over replacing it with the Willsenton, though.  

I can only share my experience with the Willenson R8 which is KT88 and EL34. I'd say you get what you pay for. The R8 used is like $1000 and that's what it sounds like. It couldn't compete with the Mastersound, AudioNote, Leben, Luxman tube amps I've owned- but it's not priced like them either.

Maybe consider a used Line Magnetic? The importer cleans up on these but the used value seems more in line with what you are looking to spend and I bet the quality is better.

Just my experience, I sent it back within the 30 day and got a refund within 45 days. 

Stick with your Cary that's likely a downgrade or at best sidegrade.

I would try and find a used one to scratch the itch before selling the Cary.  

Keep the Cary definitely…if you are looking for something different try the First Watt amps if you have efficient speakers…

I’m convinced "Willsenton" comes from a chinese mispronunciation of "Williamson", as in the classic Williamson amplifier circuit (source: I made it up).

I assume you’re curious to try DHT SET? SET has never "done it" for me when I’ve heard it, but then I like to listen to rock at high SPLs. It’s different for sure - good for lower SPLs and simpler "audiophile" music.

Best to keep the Cary while you evaluate the new amp, and sell the loser. Of course, if you’re like me, you’ll end up collecting a lot of gear that way...