I have not heard either i am afraid. I have owned and loved speaker’s from both manufacturers.
So my comments should be considered anecdotal at best.
The kef sound is better balanced smoother in the highs and a very pleasant listen. I have always felt Kef do a good job with vocals. The speakers i had used the excellent UniQ drivers, unlike the cresta’s. But the cresta’s are a classic, i remember wanting a pair when i was at uni in the 90s!
MA are what some would call hifi shop speakers! When you first hear them they sound incredibly detailed. Very fast and you don’t miss a thing. Possibly on the brighter side compared to Kef. Some complain about listening fatigue, and general advice seems to be to pair with a laid back amp like Arcam. On the Bronze range i am not super impressed i feel they have purposely made them sound a bit closed in and ‘small’ to encourage you to buy the next range up. Certainly the Silvers are a much better sounding speaker. Of all the speakers i have owned a pair of MA silver 6 floor standers stayed the longest so far!
I finally chose Monitor Audio BX-2, I first compared it with Monitor Audio Bronze 2, the quality of Bronze 2 seems to be lower compared to Monitor Audio BX-2. What I also noticed was that the Bronze 2’s cone circumference was easier to remove (the glue wasn’t good enough) than the BX-2. While the BX-2 has better adhesive quality. The weight of the BX-2 is also heavier than the Bronze 2, I see the BX-2 feels more rigid
When compared to KEF Cresta 2, I see Cresta 2 is too old, I’m worried about the quality of the driver material