Dom malia

Hi, I have recently just bought a second hand Epiphone valve senior. In mint condition. A couple of question if anyone can help. Which extension speaker would anyone advise to add to the combo? Ohms and size of speaker. The master volume does not seem to have any effect beyond the 10 o'clock mark. After that there is no increase in volume. Is this normal or do i need to have it looked at. Cheers Dom