Stereophile claims that performance is the (only) criteria for determining class designation. KEF LS50 is one of a couple lower cost speakers in class A. I've heard the LS50 and it does sound nice, along with getting great reviews, but to think it out performs all the speakers in class B seems a stretch to me. Judging by performance alone still leaves a lot of wiggle room. Stereophile defines performance as accuracy of reproduction, but how that is determined I'm not sure. It doesn't look like they just look at the frequency response and call it a day.
Along with the F208 review in the current Stereophile issue is a review of the two-way floor standing Joseph Audio Perspective ($12999). It is basically a tower version of the stand mounted Joseph Audio Pulsar ($7,000) which is already rated as class A. I could predict that the Perspective will be assigned class A as well, though judging just by the F208 and Perspective Stereophile reviews, I'd think they performed similarly with the Perspective maybe having less bass extension.
As stated on the Joseph Audio review, there is a lot of competition among high-performance tower speakers. They specifically mention the Revel F208, along with Monitor Audio PL200, PSB's Synchrony 1, and the Vandersteen Treo. I would add Wharfedale Jade 7, Aerial 7T, and the newish Focal Arias with the flax cone drivers. Each of these has flaws, but there is a lot of good sound to choose from at relatively attainable prices.
The step up from this group (which I can't afford) is defined I think in large part by better berrilium and ribbon tweeters, as well as heavier cabinets, and higher quality crossover components.
I think the Revel F208 is in a sweet spot, price and performance wise. I can also say I haven't looked back on my choice, as I keep eyes/ears on future developments in speaker refinement.