cheers Albert for the design work.  Here it is all bundled together with the original docs and modification to the filter

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Can be something like this .

i would like to see that mate

I designed a replacement for the 15kHz filters in the Pro IV, using op-amps with gyrators emulating inductors.  It was published some years ago in EDN.  I'll dig it out and post it here.

is this it?

https://www.edn.com/brick-wall-lowpass-audio-filter-needs-no-tuning/

  when necks go down , you switsh and tune in here .

when i get some free time ill look into making this into a PCB

I'm working on an updated version of the PCB, which will be smaller than the original one I did, but has extra spaces for parallel resistors (or series in some cases) for the less usual values, so that it can alll be built with E12 series resistors (which will save constructors going shopping for some of the less usual values used).  For example, 50k 1% is difficult to find, but 100k 1% is a common value, so putting a pair in parallel will get you there. 

I'm rapidly getting a dislike of the policies of some component suppliers - small quantities (10s) are often as expensive as 100 off, and the postage charges are insane!

As far as possible, I'm trying to stick to common values and trying to re-use values (like the 6n8 used throughout the filter above).  This makes construction more economical.  I was going to redesign for 10n throughout (which would be cheaper, even at 1%), but the resistor values became horribly awkward and would need as many as seven resistors in combinations of series and parallel to achieve the values!

As usual, electronic design is a whole lot of compromises!