What center speak go with Avalon Symbol?

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.

I used to have the Symbol for my home theater. It has the same timbre and maches very well with Audio Physic Center II. When you do a test tone it's like having 3 Symbols in the front.

Avalon apparently made a Symbol center channel  - just saw one for the first time. Three ceramic drivers in the center, and two tweeters (one located on either end, at a small angle). Think these were likely made later, as this example was pictured with the black shiny tweeters (as opposed to the earlier gray ones). A single port was located in the back as well.