In the early days I was using north American 9-ply Birch plywood which wasn't too heavy and had a decent stiffness to it. The second version of my cabinets I went with 13-ply 18mm (around 3/4") Baltic Birch B/B faced on 5 sides and 36mm (around 1-1/2") on the baffle. Baltic birch is ultra stiff but very heavy. The advantage was that no sound output was used to vibrate the walls of the cabinet and no bracing is required on a 1-1.5 cubic ft. 1x12" cabinet. Baltic birch won't add coloration to the sound if well damped on 5 walls. 3M makes a very densely compressed 3/4" fiberglass sheet that is designed specifically for acoustic treatment, and is way more effective that R19 fiberglass.
The advantages of Pine are extreme light weight, easy to cut and shape, and less expensive, but, you loose a lot of sound output to cabinet vibration if it isn't well braced. And it probably adds some color to the sound....in a good way.