Dont let the THD hype fool you. I own every product THD makes and love my hotplate, I swear by it, but dont believe you could use it to make even a 15 watt amp sound cranked at bedroom volume. It just wont work for any amp. For one all tube amps sound buzzy at low volumes when the speaker(s) is/are barelly moving. A really moving speaker is a huge part of your sound at normal volumes too.
Now your definition of low volume usually depends on your living situation. If you have neibors attached to your house (like a row home/townhouse, condo or twin) even a one watt amp will be loud enouph for them to hear when cranked. Many people have built a less then 1 watt amp for that purpose. A 1 watt amp is said to still be as loud as a trumpet. Its a widely discussed topic for cranked, vintage-type tube amp lovers. Do a search.
The THD only sounds decent for volume reduction on the first few notches. minus 4 and 8 Db's are the least noticable. They will lessen the volume by about 30%. Which is still very loud if your amp is cranked. In a live situation though 30% lower makes a big difference. At home, it doesnt. 12 starts to sound just a bit harsh and funky. That still only about a 50% reduction in loudness. 16 and less is just bad sounding. Read any review. Even with the compensatory bass and treble controls it doesnt soudn good at all.
Basically any attenuator is a tool for reasonable control of loud volumes and for the EVH type setup as a load box. Dont believe it will make your amp sound cranked at extremely low volumes. It doesnt.
If you dont mind playing the amp still very loud at home, it will help control just how loud it has to be.
Ive been using one religiosly for a long time. Id describe the difference as making an extremely loud amp only very loud. Or a very loud amp just loud. Or a loud amp reasonable. Does that make sense?