A lot comes down to taste and interaction with your room, so they should be your primary concerns.
I owned a pair of LS50 Meta and although I could hear strengths in them, they left me cold overall, and I certainly felt they needed to be turned up considerably to sound their best. This was having tried them with amps including the Quad Artera Play+/Stereo pre/power combination - plenty of quality power and current delivery on tap.
Depending on the size of your room I would be inclined to give the B&W 707s2 more opportunity to shine, they're very talented speakers. However, in your position, I would also have thoughts of selling all 3 pairs and buying something along with the new amp, or switching to a powered/active solution.
What kind of sound are you looking for? What are your musical tastes?
Hi Mathew, I hate to admit this but I purchased some second hand LS 50 Metas from eBay, really on the say so of What Hi Fi's testers and their verdict of 5 stars a few years running. They're quite a nice looking speaker I suppose, however, I prefer speakers with covers on them and I had the same reaction as you. They sounded lifeless to me, everything sounded as though it was slightly muffled.
The B&W 707 S2 speakers I also purchased earlier were impressive for their size but they did seem to lack the sense of space and depth of my 602 S3 bought in 2007 and are perhaps a little bright for my taste. Maybe I'll try them out again. I believe the 706 might have suited me better because my living room where the hi-fi will go, when it's ready is a large medium sized room.
Furthermore, I am slightly miffed by the fact that KEF and B&W now produce the speakers that I purchased in China, yet they still charge top money for them.
This morning I reinstalled my older made in England B&W 602 S3 speakers as a comparison again. I'm after a sound with depth, space, detail, speed, good quality, perhaps slightly on the side of warmth but accuracy.
I had convinced myself that with all the progress made, modern speakers would have to sound better notwithstanding size. Perhaps it's driver size that matters, I don't know.
Nevertheless, I still intend to go ahead with an amplifier upgrade.
My musical tastes are classical, Rachmaninoff, Mozart, Beethoven, Vivaldi, Baroque, Saint Saens, Rontgen, and others, nothing too angular or abstract. Lots of Piano Concertos and late 60s to mid 70s rock, Motown sometimes, jazz fusion for the sake of very good drummers and bass players.