The 1001RB II puts out 460 watts at 8 ohms, more when slap and pop are done. Your cab is a entry level cab rated at 400 watts that you are now putting into heavy use. UPGRADE YOUR SPEAKER CAB while you still have one. The amp head is fine but your killing that cab with it!!
This! The 1001 is a great head, but GK's Backline stuff is kinda...eh. As stated, it's entry-level stuff. An Eden cab or something from G-K's pro line would be a huge improvement.