KEF 107 and Kube setup

I ran into some problems helping my parents connect their new TV to their old unused home theater system. The KEF 107's fill the room very well but vocals from both music and movies sound muffled.

I also found the corresponding Kube which they never used but I'm having trouble getting it to work. The only audio source is the TV connected to a Sony V555ES via digital optical audio. Connecting the Kube to the Pre Out (Front) nor the tape in/outs work. There doesn't seem to be a tape monitor button or a pre amp in so I can't figure out how to get the signal into the Kube and then back to the receiver. Thanks!

Here's a link to the receiver's manual for reference:
Kef 107’s are generally, well..invariably in need of service. It’s simply an age factor. all of that series of Kef’s are in that age bracket. The 104/2, the reference 102,and so on. All are getting to be in desperate shape, if all original and unserviced.

New woofer surrounds, a complex affair..and the tweeters need to be rebuilt, due to dried out ferrofluid. Another complex affair.

Nothing bad about Kef speakers, it’s a simple age thing. The brand’s longevity in the market makes it so.

Asses the qualities of the speakers, first. Ascertain if they meet spec. Research what I speak of. It would be very unusual if they did function 100%. download the manual for the speakers and kube.

the youngest the speaker could possibly be is +25 years old as old as ~30 years.

eg, here's a video of the complexities of dealing with the tweeter.

The Sony does not have a power amp in, so you would need to connect the cube into the tape monitor loop. There should be a way the optical input would run through the tape loop ( possible ). Because the source is tv only, it is possible that all of the "voice" info is being broadcast in "multi channel" mode, making an assumption that there is a center channel being used. The voice is lowered quite a bit from the mains, and directed towards the center. I do not believe the Kefs are in need of service, if, they have been stored properly ( room temperature and such ), and not under or over driven by an amplifier. Just my thoughts, but I may be wrong.
Some AV receiver can’t use tape loop for digital input sources. If your TV has analog audio output ( usually a 3.5mm phone jack ), connect the TV via a 3.5mm to rca cable instead of digital optical.

Maybe you can use the internal AM/FM tuner to test the Kube via Dat or Tape loop.
@imhififan , exactly why I said " possible ". Good idea with the analog audio out !
Well, you could attempt to use analog outputs from the TV and put the Kube there. Alternatively, I did notice this receiver has some very limited EQ (glorified tone controls) which you might be able to use to fill in.

One other option is to use a receiver with full room correction, which will have better ability to adjust the receiver for the lack of a Kube.
Thanks for the tip! I took the top hat cover off and gave it a listen and there isn't any sound coming from the tweeter. I guess I'll look into repairing or replacing the tweeter.