About a year ago, I purchased a used SVT-IV Pro bass head (older model with the red check for a V) and have since used it running mono-bridged with a Neutrik cable to a Warwick 6x10 cab (6 ohms). For the extent of me owning this head, it has worked completely fine and has given me no issues until today. Now, whenever I turn the amplifier on, if there's any sound input at all, the yellow limiting led lights up and cuts any audio from coming out. I have my eq flat, all the other knobs at noon besides master volume, ultra lo/hi off, and bright off. I increase the gain so that most every note makes the red clipping led light up, but the slightest bit of signal gets limited by the time it hits the post amp. With limit bypass on, I can get a signal that seems as loud as it should be, however, it's terribly distorted beyond any natural, intended sound. (The amp has never sounded this distorted, I'd have to crank the gain for a simple, small crunch/bite in the past.) I power the amp through a Monster Pro 3500 power system on the high amperage outlets and have tried switching outlets with no luck. I have also tried using a standard 1/4" speaker cable on both the A and B amp channels while in stereo mode with no luck. In the past, I have ran audio directly from my computers sound card to the amp using a 1/4" -> 1/8" adapter to add extra bass to my sound system. I have also hooked the preamp out of my Peavey 6505+ to the input of my SVT-IV PRO to see how the bass speakers would handle the distortion since I tune very low on guitar (kinda like a bi-amp setup, but with no pass filters). I have no idea if these actions were damaging to the SVT-IV PRO, but it has worked throughout this with me using it to play bass in between fine. I'm not sure what else to try and I have searched these forums for similar issues- the main difference I notice with my issue is that the volume is limited to the point that I can hear the strings themselves louder than what's coming out of the speakers, it's almost a full cut, and when I bypass the limiter, the tone is unnaturally distorted. Any insight into what the problem is would be helpful; I am willing to pay to get it fixed once I have the money, but if it's as simple as changing the preamp tubes that won't be needed. Thanks in advance.