KEF 105/2 center speaker suggestion?

I would look at some of the smaller KEFs from the same period. I had the 105s years ago but that was before multichannel. The tweeter and mid range of the smaller ones should be similar to your mains to the best of my memory.

I have switched back and forth between KEF 105 2's and 105 3's as my stereo speakers and the fronts to my home theater set up. I am presently using a Monitor Audio center. IMO a Monitor Audio center (say a Silver), a B&W (say the HTM 62), or mid priced KEF center should suit you well. If you want to move away from the British lines, I have heard nothing but good things about the Revel c-12 when matched with many British speakers.

I second the use of a pair of B110 based mid range book shelf speakers like the Cantata, Concerta, Cresta, Coda I&II, etc, put in parallel. Next in line would the more modern B200 based mid ranged speakers. If space and wife permits, best use the excellent mid/tweeter/mid section and the cross over part of a dented/cheaply bought KEF 104/2 or just plainly place the entire speaker horizontally.

I recently changed my 5.1 HT setup from Maggies/Snell Acoustics to a fully restored KEF 107(refoamed all woofers, replaced most caps in the crossovers with polypropylene caps with metal film, resoldered all on boards and connectors with Cardas silver solder and a lot of wood staining) as fronts, KEF 104/2 as center and surrounds (it took me a while to match the wood/finishing and colors of all speakers).

All that out of love for great sound and KEF nostalgia.