Yes you can set them however you want and yes this can damage speakers. These are just volume controls but unlike the normal volume control they only change certain frequency ranges. Just like you can damage your speakers by turning the volume too high with the normal volume knob, you can damage them by turning either of these volume knobs too high.
Turning up +15 dB is a lot. Turning up the bass for example by +15 dB increases the power demand by a factor of about 31.6x. So if you have an amplifier capable of 100 watts and you want to have mid frequencies playing at a level that requires 3 watts, your bass frequencies would require 95 watts to keep up assuming equal input signals. Since music usually doesn't have equal input signals at all frequencies, more likely your bass demands are already far exceeding the amplifiers 100 watt capability while mid frequencies are only getting 3 watts.
Your amplifier only has so much output capability. Volume knobs are also called gain control. You have an input signal at some level and you multiply that by the gain and get some output level. If your maximum output is 28.3 Vrms (translates to 100 watts in 8 ohms) and you have an input level of 0.283 Vrms times a gain of 100, you are at the maximum capability of your amplifier. How you reach that gain of 100 is a function of all the gain control knobs which includes the volume, the bass, the treble, any "Loudness boost" etc. If you turn the bass way up you have to keep the volume way down to avoid the total gain in the bass region exceeding the maximum. Same with treble.
