I've just taken ownership of a Katana 50w mk1 that I bought on eBay. initial impressions are that it's great with loads of flexibility and features, especially for a home player such as myself. However, the moment I push it past the first couple of notches on the 0.25w clean channel, I get a notieable humming sound, and when I turn on the crunch channel or push up wattage, it just gets louder and more intrusive.[br]I've tested the amp down at my work, just to see if it's my home's power supply, but the noise is still there. It tends to diminish when I hold the guitar strings, which makes me think it's a grounding issue. Is there a straightforward fix to this or should I look to send this back to the seller?