Something I just started to look into is the effect of the transformer on frequency response. The low frequency response is set by the reflected impedance at the primary in parallel with the output tube plate impedance then that being in parallel with the primary inductance of the output transformer. Bottom line the greater the primary inductance of the transformer the lower the frequency response so in simple terms a bigger output transformer will give you more bass response. Also if you lower the reflected impedance you will increase the bass if you connect a 4 ohm speaker to the 8 ohm output the reflected impedance will half so 2500 ohms instead of 5000 ohms so if bass response was down to 100Hz it would drop down to 50Hz. What the triode mode does is change the plate impedance to 2k ohms from say 50k ohms so with 5k ohms reflected in the pentode mode you have 4.5k in parallel with the transformer impedance while in the triode mode it would be 1.4k ohms. So if you’re bass response was down to 100Hz in the pentode mode it would drop to 30Hz in the triode mode. So this is another reason the triode mode will sound darker.
Another way to increase bass response is adding feedback to the amplifier. The triode mode is self inflected feedback because the control grid is not isolated from seeing the output plate as it would be in the pentode mode.

Hope this does not muddy the waters of understanding what’s going on in your tube amp.