I think you've already spelled the options out.
Personally, I'm waiting on TLS Guy to chime in, as he seems to be extremely familiar with KEF's history (among other English speaker manufacturers).
Man, I googled your mains, and after seeing what they look like, I don't doubt that your center can't keep up, or is matched.
As for phantom not working well enough, are you assuming/guessing this to be the case, or have you actually tried?
One nice thing about KEF for mch setups involving a horiz center is the coaxial design to avoid lobing.
Sorry I didn't really help, but I wish you luck!
You would say that wouldn't you?
Your speakers are Raymond Cooke's last big design, before his untimely death.
He, or a very senior engineer personally checked each pair of those to a company reference pair.
They are excellent speakers still, though like all speakers not perfect. They have a nice reserve about them, and a good smooth sound stage.
Raymond loved plastic coned speakers, and pioneered them. His first, the still sort after and still highly useful, polystyrene coned KEF B 139 bass driver. I have six in use right now and a pair in reserve. He pioneered the Bextrene coned speaker. Your speakers have Bextrene cones. Bextrene is a plastic no longer produced anywhere now. In some ways it is a good material. However it starts to break up around 1500 Hz, which makes crossover design a headache. I'm not convinced Raymond got the Bextrene character out of those speakers completely, though it is well tamed.
KEF have now been owned for some considerable time by Gold Peak industries out of Honk Kong, and most of their output would make Raymond turn in his grave. I recently auditioned some KEF coaxial speakers, and a pervasive feeling of sadness came over me for Raymond's legacy. He was a friend and in every way a mentor to me. He lived within 15 miles.
So I can certainly see why your current centers stick out like a sore thumb, and it is not in the favor of the newer speakers.
I think you might regret getting rid of your 105.4s, especially if money is tight. You will have to spend about 4K to get anything that will equal those.
So, I would first try a phantom center.
If that fails your best bet is to find a pair of BBC LS3/5a. This is the most famous of the designs that came as a result of a big research program into improving loudspeaker performance back in the fifties at the BBC. The fruits of this program, were to make British speaker designers preeminent, a position they have never lost in my view.
Now both Raymond Cooke and Jim Rogers were active participants in all of this. In those days in the UK, everybody got along, and improvement of the art truly was ahead of making your fortune. Trade secrets did not exist in that golden age, I remember so well. Every year we would gather at the Hotel Russel in Holborn for the Audio Fair. Everybody discussed their latest ideas and endeavors openly, and helpful suggestions freely offered. I'm pretty sure Stan Kelly got the idea of suspending the stylus of the Decca ffss from us. I and my father showed him mono moving coil cartridges with the stylus suspended on nylon. They tracked at 3 gm, when everything else was tracking at 10 gm. When you are a kid you have nice small hands, and if you are patient you can work wonders.
Jim Rogers and Raymond got to produce the LS3/5A
Many of the BBC engineers involved in that research, went on to found loudspeaker companies, like Castle, Harbeth and Spendor. Harbeth, Spendor and Goodmans also produced the LS3/5A. It is a fine speaker, but the crossover is about the most complex on earth, to deal with the Bextrene problem
This speaker was used extensively by the BBC for outside broadcast monitoring applications and in house production. It is an iconic product. There were a lot produced and a lot still around. However they are highly sought after.
Your best bet is to find a set of those LS3/5As and use one as your center channel. It has a Bextrene cone, and comes from the same body of work as your current speakers. It will be nigh on a perfect match. Make sure it is vertical, do not put it on its side.
Expect to pay over $1000 for a good pair. You might well recoup a good chunk, or all of your investment, by selling the speaker you don't use. Chances are good by now someone has a pair of these with one speaker that does not work, and is missing his speakers, and will pay mightily to get them back.
This set by Jim Rogers, just sold for $1327.
Another set by Jim Rogers is available now, but the location is Ireland.
There are no bids yet, so the owner might work with you.
It seems the later version of the LS3/A by KEF is the best one to have.
