There are several things you can do. Cheapest is putting a lower gain preamp tube in the first preamp position, something like a 5751, or even a 12AY7 for even less preamp gain. You can change the speaker to a very efficient (=loud) one, that'll get you louder cleans too. I'd recommend something like a Jensen Neo - which has the added bonus of weighing next to nothing for such a loud speaker. Also, you can put a bigger output transformer in the amp, take a look at Allen Amps' website, they have one specifically for that purpose.