Jake,
If you decide to get an attenuator, please check out the tube cube I listed above. The guy who makes them is a great electronics engineer and a musician, he built the tube cube precisely because the THDs and Powerbrakes were so god-awful expensive, and they really don't need to be. Besides that, all the THDs are one-trick-ponies - they only work for one ohms setting - 2, 4, 8, 16 - the tubecube is fully adjustable for all ohms settings, and has a volume control that allows you to dial in exactly how loud you want to be, from bedroom quiet to full-on shred, without coloring the sound at all. I can attest to this, as I use one in my Marshall Plexi 18W clone - it is just unreasonably loud without the attenuator, but with it installed, I can dial the volume level I need without changing the tone at all. I can get that blistering tube-driven distortion, but at any volume. I should mention that I am not, in any way, affiliated with the company - I'm just a super satisfied customer. I have mine installed between amp and speaker, inside the back of my cabinet, and now it is suitable for bedroom, barroom, or stadium.
Dig it!
Rob