Tune Whites Coinmaster coil

[Old cart]

Just picked up a Whites coinmaster (current model like Prizm). Decent detector but does not go very deep. I understand it uses an untuned coil. I also understand you can use a tuned coil from a more expensive detector but this sort of defeats the purpose of a cheap detector, those coils cost more than the whole detector costs. Is it possible to tune this coil to increase depth? I think just adding a correctly sized cap, maybe in the connector, to the RX windings.
If it is possible is and a good idea any suggestions other than just tune slightly below the TX frequency? I have tools to measure RX gain phase characteristics if that helps.
I was also going to adjust the GB and add an external threshold adjust. Any other ideas?

[Funfinder]

First it would be good to get info if identical models of that detector really have more depth at all
and if it's the original coil.

If you have some variable capacitor in the pF range (5 to 200 or something) you can
connect it where the coil-cable plug is connected and try if you get more depth.
But check if the discrimination values are still remaining the same as before or if
the ground-balance changes.

Another good idea is to open the coil, check out the windings and create a larger coil,
because often those older detectors have pretty small coils which aren't very deep.

[Old cart]

I may try that. I did adjust the threshold up until it was barely audible. I also adjusted the ground balance to my local ground conditions. This was significantly different from where the factory had it set. It seems to go significantly deeper but I have not yet tested it out under controlled conditions in a test bed.