used Integrated tube amp under $1500 for Klipsch Cornwall III's?

You might be able to find a used Audio Research VSI 55 for around that. I have one and have used it with my Heresy 1 speakers and it sounded great. I'm currently listening to my Focal electra be 1008 speakers with the VSI 55 and it's sublime.

It would help to know the size of your room, music preference, and how loud you like to listen.. There are a lot of good choices.. If you are in a moderate size room listening at moderate levels, the Coincident Dynamo 34SE MK III amplifier could be really nice.. the mkII has received nice reviews ( Editor’s Choice TAS, Blue Note Award by  enjoythemusic, Positive Feedback ) and the mkIII has some nice upgrades

Pretty much anything will have enough power for the Cornwalls.  At  102db/w sensitivity, even a single ended triode will push them to close to 110db on peaks.  For the OP, looking at the features and sound characteristics of the amp are far more important than power output.  

the room I'm not entirely sure how to define, kind of an open concept condo living room maybe 10ft wide and 30 ft long leading into other open areas...kitchen/dining area/hallway
hardwood floors, no carpeting, speakers will be positioned in the corners (replacing Forte III's)
I'm going to get a 10x6 rug to place the speakers on
listen to all kinds of music, will be using various digital sources and looking to upgrade that in the future but primarily for the time being an LG V30 connected directly to amp

that vsi55 sold used here for a steal not too long ago I think around $1500 canadian and was sold in like one day, lol
I've accepted that this will not be my last amp, over time want to try out a collection of different tube amp types and maybe some solid state

Decware would be an excellent choice! US made and lifetime warranty. More importantly sounds great