The SVT's were made for (at the time) standard LB, and while they can handle some HB, it will beat them like a piņata, and reduce the life of the rifle and it's parts. I stay with nothing but LB, and have had best results (with the adjustable gas system) with moderate reloads.
That being said, stay away from Polish LB in an SVT. I had very bad results from some of this, resulting in smoke and flying extractor, plunger and plunger spring. I have a few cases of it among others, and will probably pull down most of it for components, as long as it doesn't pass muster in my Mosins. I've attached two pics of Polish LB cases that were shot in my SVT. Notice the longitudinal failures along the case, with the second one suffering almost a complete case split.