Sounds like it may be a dodgy unit, but before returning it, double check all cables and bass control harness & jack socket wiring etc, dodgy or dry solder joints can play havoc with signal path
I have a Hartke LX8500, ( I also had the LX5500, but didn't need both) I think they are great, plenty loud, easy to use and probably the best value 800W class D out there. I like the tone and punch, the compressor works well for me, I go for a clean tone and tend not to use overdrive. I love the backlight controls on dark stages I have been using this head with a Barefaced Super 12T Or my Vanderkleys. The only minor issue which does not affect or bother me is the fan cycling on and off which is audible in studio but inaudible when gigging. I regularly use the LX8500 in preference to GK, Mesa, GR amps costing twice as much.