GK 700RB head has had some scratchy sounds when operating the master volume. Sometimes my volume will get low for a few seconds while I'm playing and then come back to normal. At practice last weekend the speakers started to "crack" like a jack was bad or a cable was loose. (which didnt appear to be the case.)..making me wonder if the speakers were the issue..I'm running into a 90s era Fender Bassman 410 (with a blown horn turned all the way down) and a 115...also two nearly new cables and one tuner pedal..if that helps at all. Any thoughts? Thanks.