Is it a set up problem?

[Arnold Schnitzer]

Please check the soundpost position. It should be about the same distance in from the g-side f-hole as the bass bar is from the e-side f-hole. It should be about 12-20mm down from the bridge. It should fit really well and be just a tad snug with the strings off. A larger diameter post will likely make the bass response worse, not better. If you want to boost the bottom, move the post out toward the f-hole a bit. I fear that Ken is probably right, though you state that the arco tone is deep. How high are the strings? Is the nut cut nice and low?