I wouldn't worry about it, it's just a joke of sorts. On the back of the amp, Warwick have written the details, using the font (police, typeface, character set...) 'Comic Sans'. There's nothing wrong with that; it's just lightweight humour making fun of having chosen that font instead of the more 'formal', 'serious', 'severe' fonts usually seen on equipment. It's no reflection on the qualities of the amp, nor of your ad; just 'British humour', that's all. Chuckle and move on, I'd say.
Good Luck With The Sale.