1000W with a Low B4 sounds about right. I used to run a Low B2 with about half that much. Whether at Low B4 will work well with a 300W tube head depends on how loud you need to play and the sound you want. The sensitivity for the Low B4 is listed as 96dB 1W/1M.
IMHO, the Low Bs are not super tight even with a big solid state amp. I suspect you might get a woolly mess in the low end with an all tube tube head due to reduced damping.