What tube would be a good replacement for either an EL84 or a 12AX7 in order to achieve a more distorted tone?

I have just bought a peavey clic 30 and found that it doesn't really distort that much at all unless you turn both the volume and gain on the distorted channel to full. A friend of mine suggested that getting different tubes might liven up the tone without the need for a distortion pedal. Can anyone suggest anything?