Ideas for creating a nice reverb effect for a vst synth instruments would be welcome what opcode and what parameters 
Reverb for virtual synth
You should send the output of the synth through an always on instrument rather than adding it directly to the synth instrument. reverbsc is a pretty good reverb that was written by Sean Costello of Valhalla plugin fame.
Thanks the reverbsc works wonderfully 
another question cabbage related how are the bounds on a widget set eg bounds(96, 80,100,200) which way is that formatted
in your example :
x=96, y=80, width=100, height= 200
more help available here: https://cabbageaudio.com/docs/button/#bounds
Is the madsr opcode a good choice for a filter envelope
Sure, ADSRs have been controlling filters for as long as I can remember 
Some classic synths use slight variations on the ADSR idea, but for the most part you can’t go far wrong with an ADSR, or madsr in this case.
Where would you implement an adsr into a moogladder would asig moogladder A1, KFC*adsr, kres
Make scence
That should work. See my other post.