Proac reference 8 signature or Meadowlark Kestrel

Good point from ZK. I am referring to the original Kestrel. To be specific, I have owned the original Kestrel, the Kestrel Hot Rod, The Kestrel II, and the Shearwater Hot Rod. Of those four models the Kestrel II, while the most attractive, is my least favorite. I sold my Shearwater HRs when we moved into a small apartment while a home was being built. I would purchase another pair if I could find a pair within driving distance.
As for the difference between the original Kestrel and the hot rod version, I could live with either.

Those are two very different speakers you are asking about. I have owned Kestrels for about six years now and have tried numerous times to upgrade from them. One of those attempts was with a pair of Proac Response 1.5s. The Proacs were excellent speakers with good bottom end and plenty of detail. What they lacked was the musicality of the Kestrels.
If you look at my system page you will see the various speakers I have tried to replace the Kestrels with. In a small room, with modest electronincs, it is very hard to beat the Kestrels. They sound more like music, not a stereo playing, but music to me than any other speaker I have owned.
Although I only briefly used them with tubes, I can't say how they would work with your amp. My understanding is they are good with tubes. I can say they maintain their tonal balance at low volumes very well.
The problem is finding a pair. And if you do find a pair, what condition they are in as they are going to be ten years old. They use time aligned drivers and simple 1st order crossovers with very high quality parts so if they were treated well they should last a while. Good luck whichever way you go.c

The Kestrel II is certainly a better looking speaker than the original Kestrel and to many people may sound better. I owned the Kestrel II for about six months and really tried to like it but it just didn't excite me. The sound was good and maybe had a more audiophile quality to it. To me that is a more sterile sound. It seemed to portray the music without the liveliness of the original.
Like so many other speakers, when I put the original Kestrels back in the system to compare I had that "now that sounds like music" feeling. What I like about the original Kestrel is its natural sound. I describe the original Kestrel as "warm." They have plenty of detail but never fatiguing. Another aspect of the Meadowlark speakers is their ability to deliver a three dimensional soundstage. Both models do this very well.
I had a similar feeling about the Proac 1.5s but to a lesser degree. Choosing between the Proacs and the Kestrel II is a much moree difficult choice. Both seem very well made with a slight edge to the Kestrel II.
Of course what really matters is your ears and your sonic preference. I would go with the Kestrels becuase I love the three dimensionality the time-aligned drivers gives. Probably hard to go wrong either way.
Good luck and please let us know which way you go.