Electro Harmonix 15 Watt Howitzer Pedal Amp/Pre

I used it with a 15 W 8 OHMS speaker. When topping the volume, it introduced unwanted distortion even on Return input. I play through Fractal Audio FM9 and also stompboxes. I compared it with an older 15 W amp on 8 OHMS in the same combo, and somehow with the same speaker, EH was quieter. My unit might have malfunctioned because, since it doesn't have any power switch, every time I inserted a power adaptor, it threw sparks! Bad luck. Returned. Thomann refunded okay.