If you don't have a blender of sufficient size, or you desire a thicker soup, you can use a potato masher on the soup in the pot rather than pureeing the vegetables.
The last time I made this I used a bit of cayenne, a bit of chili powder, a bit of Louisiana Hot Sauce and a bit of Siracha. The mixture gave it a very good, slow-building heat.
Adapted from Sundays at Moosewood Restaurant