My tiny terror sounds the same volume when switchin between 7 or 15 watt settings

Well, the thing to shift between those is more of a marketing ploy than a useful function. The difference from 7 to 15 is ONLY 3 Db... which is about the minimum step one can hear in sound level... The amp might be able to deliver more power at the 15 setting than it can at the 7, but you have to crank it up to do it...