I know little about the Avalons, but if the S3/5 is a BBC-licensed minimonitor like the Rogers LS3/5A, it's rather thin sounding, or at least has very little output from the middle of the midbass down. That already is a mismatch with the Avalons, I believe.
But to your question. The Aerial Acoustics center speakers are VERY well regarded. The original $1200RR CC3 'won' a center-channel-speaker comparison in, I believe, 'The Perfect Vision' years ago, and it's been upgraded to the CC3B ($1600RR) since. I have a CC3B running with a pair of old-but-still-great Kindel PLS-As* and am VERY happy with it and the combination. The bigger-and-later CC5 (about $3500RR, I believe) has slightly larger woofers and is more extended in the bottom octaves.
Try one; you may love it.
*--Tall, big-sounding speakers that are somewhat-warmly balanced, with good bottom-octave power.