Running dual sub outs at line level plus low input impedance amp
will prove fruitless for you and is suicide for any fine tube pre amp,
Try going out with subs at speaker level and find an pre owned Atlas.
My amp (Sanders Magtech) has 100kohm input impedance. Is that considered low input impedance? I'm not sure that I understand your post about subs. My subs are powered, and I've read that it's better to run them via line out and not through speaker level.