It produces its full 300W through 4 Ohms, which means you need to add another speaker cab which has to be rated at no less than 150W and is 8 Ohms.
Currently, through 8 Ohms, it's giving in the region of 150 - 180W. Adding the second cab will give a large increase in perceived volume.
You can add another 1x15 or a 4x10 (or even a 2x12, but that would need to be 2x 16 Ohms in parallel - I'm not sure how common they are).
When you have 2 cabs of equal resistance (in this case, 8 Ohms) the output power is divided equally between them. So 300/2 means a maximum of 150W into each cab.
G.