Hi. I've bought today an Hartke 3500 HA and an Hartke 4.5 xl, all new. In the amp, that has 2 channels, the tuve channel and the solid state channel, when I play only with the solid state (transistor) there's no problem, and no noise in the back. But when y play with the tube channel, or with both of them, there is a lot of noise in the back, and, besides that noise, there is a "crack pfff crack" noise, like if something is crunching... Is it normal? And why in the tube channel?