The main claim to fame of fixed bias, is efficiency, you get more power out of a set of tubes if you are not heating up a cathode resistor. But with FB you are applying a voltage, aka (signal) to the grid. If this is not deep black it will be heard.

With CB you will have a certain amount of degenerative feedback , which is not a bad thing and my guess is that is what you ( and me ) are hearing

With CB often you are running you tubes deep into "class A". You can test this. Measure current across the cathode resistor . then when you switch to FB set the current to the same or more.
Try this and tell me what you think…John