Since the SF Vibrosonic Reverb / EV 15" doesn’t have a horn or tweeter, then I think Rich is losing out on a lot of the high end sparkle which really helps to make the acoustic illusion. Are both pup systems going to only 1 amp? When I engage the piezos I also use it in conjunction with my mag sound as well (2 separate amps and or tracks). The mags add body to the tone. With the extended lows and highs of the piezos it makes a nice sandwich. There are also a lot of newer amps designed for acc guitar and these always have high frequency speakers beside the woofer, which tells you something.
But just to isolate the problem he is having with crackling it is easy to bypass the preamp to trouble shoot where the noise is coming from.
With a preamp / vol knob and a high pass filter the Tele controls must be getting a little crowded no? On my guitar I like to keep it traditional looking as well as having that extra room for more mag tonal options.
I used to have a clever little saying that goes here, but I lost it!