You can check Falcon Acoustics UK.
Recent Crossover Overhaul:
My friend has two identical sets of JSE Infinite Slope Model 2’s. They sounded the same to both of us in repeated comparisons.
OLD: he overhauled the crossovers of one pair, we compared last week.
Cassandra Wilson; Kate Wolf; Annie Lenox; Richard Burton, VOICES
unchanged pair: maintained the voices as I/we know them for many years
altered pair: ALL the voices were shifted slightly up, the voices slightly ’wrong’, Richard Burton and Cassandra Wilson, their voices are very revealing, I always take them to stores and to friends systems.
Two adjustable crossovers, NICE BUT TRICKY:
Unless you know crossover parts have failed;
Before you mess with crossover parts (no matter how old they are):
I would get a sound level meter with tripod mount,
AND a test CD (not lp) with individual frequencies.
Tripod, seated position, seated ear height.
You say midbass is bad: Find what you are getting now. Mess with your equalizers, see the effects via the meters, i.e. actual not apparent.
TRICKY: To adjust, you need to match the l and r individually to each other, AND, you need to equalize the room when both are playing.
I have level controls, simply volume of mid horn to woofer, then volume of tweeter horn to mid horn, back and forth, again/repeat.
btw, a meter will hear objectively, does not account for your preferences and or hearing, i.e. old dog needs to boost the highs a bit more than the meter shows.
"Mine has 2 crossovers. One inside the mid and the other in the bass box. There’s another board for the knob which I wonder if I can disconnect."
I'm just guessing, in case you could 'fix/adjust' your speakers before upgrading parts.
Is it possible you do not have the KUBE 200?
this one is sold, but it shows what it looks like.
"Our test speakers were furnished with an active external equalizer, the KEF Kube 200, designed for use with the Model 105/3’s.
The Kube 200 should be installed in a low-level signal path such as a tape-recording loop or between a preamplifier and power amplifier. Pushbuttons on its panel can replace the tape-monitoring function (there are tape-in/out jacks on its rear apron) and bypass its equalizing circuits. It has separate low- and high-frequency control knobs, with center detents, and a small, separate power supply.
The Kube 200 is 6 inches wide, 8-1/2 inches deep, and 2-1/2 inches high. Price: KEF 105/3 speakers, $3,500 a pair; Kube 200 equalizer, $390."
IF you proceed:
Normally, each part, capacitors/resistors has identifying marks, and you can get new parts with those values (i.e. +/- 10%, or 'better' i.e. tighter tolerances.
My friend bought a device to measure his crossover parts, found a few parts that were bad.
I can ask him which device he bought if you want.