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.

I had been using a Parasound A21 with LS50s in a moderate size room. Cary CAD 572 SE 20-watt monoblocks that were loaned to me generated a soundstage with surprisingly greater depth and detail. I was hooked on that sound, I’m currently using an Ayre A7e integrated with 60 watts @ 8Ω, 120 watts @ 4Ω in that setup. The sound seems to have much the same quality, but the soundstage seems not quite as deep as I remembered it to be with the tube amp. On the other hand, there are no PITA tubes either.