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

Grid bias exhibits lower distortion (and more power, as Forty2wo noted) than cathode bias does, and the orders of distortion artifacts present with each differ. Output Z and therefore damping factor as well also vary with the bias method, and whether or not the cathode resistor is bypassed. With a cathode biased stage there is indeed degenerative feedback, but the parallel bypass cap provides a low impedance AC path to ground and breaks it. Adding the bypass cap increases gain, distortion, and stabilizes the bias effect.

A grid biased stage can be either Class A or Class AB operating class. A cathode biased stage found in a hi-fi amp is typically gonna be Class A, while a cathode biased PP guitar amp may be either class. The reason is that with Class AB operation the plate current increases significantly with output, and an increase in current drawn through the cathode resistor creates a higher bias voltage, according to Ohm's Law. If an amp is (cathode) biased into Class AB operation far enough away from Class A, it will increase it's bias voltage on the fly enough with increased output wattage to the point where crossover notch distortion becomes horrendous. As you increase the volume further, it'll sound more and more distorted until it actually starts losing volume from the power tubes being cut off!

That's of course worthless for hi-fi. But, there are a few guitar amps like the legendary Vox AC30 (and boutique clones) that use a cathode biased Class AB output stage to produce a unique set of harmonics as the amp is turned up. The AC30 is biased close to Class A operation, so the power tubes' current draw doesn't increase really sharply with output, which keeps it out of trouble.

Another difference with cathode and grid bias is that with cathode bias, the plate voltage you measure is NOT the real plate voltage. The real plate voltage is the plate to cathode voltage potential. Subtract the cathode voltage from the measured plate voltage, and you have the real plate voltage. This must be taken into account when designing a cathode biased stage on the plate characteristic curves.

Thermionic

Class A circuits have the active component always conducting, never cutoff even at idle. Class B is push-pull where each device is set for zero idle current, so that it only begins to conduct on its half cycle. Class AB is a push-pull configuration that sets each active device quiescent current above zero, so each device conducts through the crossover region. It prevents switching spikes and reduces distortion. So Class A is conductive all the time, Class B is cutoff at opposite half cycles and Class AB is somewhere in between.

The whole idea of single-ended circuits is to set the active element in the middle of its linear range, basically right at its 50% current point at idle. Then the input will swing current above and below that, with maximum output being near the point where the device is completely saturated on the positive side and completely cutoff on the negative. If the idle current is too far off 50% either way, you'll hit one of the limits considerably before the other one, wasting some of the dynamic range potential.

For a pentode, I don’t know ask someone else.;)

Now if you bypass the cathode resistor with a big cap. From a AC (impedance) point of view it’s not there. So you are more or less back were you started, from a Z-out point of view.

So is any of this to worry about. No. there is not much you can change, so go with what you like and don’t worry…John