We used that Yammie effect box back in the late 80's, early 90's
While it was pretty good for its purpose, we struggled with getting some sounds we were after. This may be the case with you. It has some neat sounds, especially for electric guitar, but is not an 'ace of all trades' box in my opinion.

If I remember correctly, it doesn't have an XLR input so you must be using the pickups, which may be part of your struggle. That sound may be easier to get by using a mic, so you can control the body resonance easier.
Another thought - I can email the guy who owns the one we used, as he is a guitar player too, and see if he has anything programmed in his already that is close.

Make your sound your own!
.. I do not work here, but the benefits are still awesome