Physical Modeling: Guitar, Piano, Bass

Hey everyone!

I am in a beginner max/msp class. Our final project is to do physical modeling of guitar, piano, and bass. My partner and I have been working on this constantly for the last week and a half, and have gotten nowhere. We were looking at using biquads~ to make filters, but our class never covered it, and it seems like there is an easier way to go. My main questions are:

Why is it when we remove the biquad, no signal is passed through at all. Biquad is just a filter, right?

The biquads appear to be controlling most of how the timbre of the guitar is formed, so is there anywhere that talks about piano and bass parameters for a biquad, or what is another way of forming these timbres? I feel that the filtergraph~ and amplitude envelope don't have as much of an impact on the sound.

Once in a while, our sound stops working even when we won't change anything about the patch. Anyone see a solution?

Check out our patch, and any help would be greatly appreciated! Thanks!