Compared to shimano mine seem really stiff. Is there anything I can do to loosen them up?

Chances are its the cables are really stiff not the shifter itself. Replaces the cables (both inner and outer) and see what they are like.

all the cabling is pretty new, and isn't too twangy

How stiff is really stiff? Compared to the shimanos are they twice as stiff or what?

stiff enough that i've sprained a thumb shifting – at least twice as much resistance as the shimanos

Your cables are shitted up then.
Replace them with Shimano SP41 outer and a good inner.

It you're sure it's new cables then open it up and take a look. I had to do mine today on the trail because some grit must have got in there and was stopping it downshifting and making upshifts very stiff. Cleaned, a drop of lube and screw back together. It couldn't be easier.

That doesn't sound right, something's stuck ia housing somewhere, or you've a break in the out causing it to compres and foul the cable.

x9s are a little firmer than Shimano, but more like 'crisp' than 'injury causing'.

Thanks for the pointers so far.
I'll whip the covers off tonight. They are on a pretty much new bike which had the cables properly cut & fitted. Levers have always been stiff – I was hoping they would loosen up with use…