The bass is being amplified by a Schertler Dyn-B, which is a contact “mic”, not a traditional piezo. Although I believe it has piezo crystals in it. Not any sort of ribbon. It also has an AMT sb-25 mic on it.
Generally don’t have problems with feedback at the volume levels I’ve used with the bass..
The double soundpost is structural, as the top was collapsing somewhat under the pressure of the strings, perhaps this is a design flaw in the instrument. It came with a set of really cheap, thin Chinese upright strings, now it has Spiro 1/4 size, which only come in one gauge... medium. also, it was designed , supposedly with the option to use E-bass or upright strings. It had two tailpieces, of different lengths. Chennell Obviously, never put quality upright strings on the bass, or he would have noticed the table collapsing.
I understand archtop guitars often have two soundposts, however, as a device to minimize feedback. Perhaps they function similarly in this bass.
The bass is 34”,Same as an E- bass. Actually it is closer to 33 1/3 inches. Slightly shorter.
Position on the body greatly effects the timbre of the signal of the Schertler dyn. B. Obviously, it wouldn’t work on a slab body
Why do you need two microphones, and which of the two microphones on the instrument do you think is most important? For example, could the Schertler Dyn B be enough to carry the acoustic tone without the AMT sb-25?