KEF LS50 Meta and 40-ish WPC Tube Amps

It will depend on your preferred listening levels, type of music, and the size of your room.  If you're setting up in a smaller room and not looking to blast the volume, then something like the Primaluna should work well. However, if you're in a larger room and listening to symphonies and EDM at high levels, then you'll probably be disappointed.   

It would work, but you're pretty close in price point to a Rogue Cronus Magnum III that will give you 100 watts per channel.  It's a nice match with the METAS.  (I sell both Rogue and KEF as a disclaimer)

I have metas but no, not used a tube amp with those or prior ls50s.

I did use a highly regarded 180w/ch zero feedback SS amp (higher output impedance like a tube amp) with ls 50s the first time I ever heard them when another Agoner brought a pair over to listen and we were both quite unimpressed. THen hooked up to my current Class D SS amps and it was night and day.....very good. So I can vouch they can be very sensitive to amp used and are NOT designed to be used with tube amps so I would personally not go there with any ls50 for best results.