There isn't much energy in a bass guitar at this low and high frequency and some engineers simply apply high and low pass filtering. I have these two frequencies on my Trace Elliot AH-1200 amp and have tried taking them out completely. I think the 30 Hz rumble taken out sounds cleaner and less boomy but I'm not 100% sure whether to remove the 15 KHz frequency 'cause it takes away some of the brightness, albeit hardly noticeable especially when compared to other treble frequencies such as the 5 KHz. So, I was curious as to what other bassists do with these frequencies, if at all they have them on their amps because these are quite extreme frequencies! One last thing about the 15 kHz. One could argue that by taking it out, since it's practically beyond the range of a bass guitar and more belonging to things like cymbals, that by taking it out, one is making space for cymbals in the mix. Conversely, low frequencies are usually filtered out on cymbals, so even though one may lose a bit of 'shine' on the bass by taking the 15 kHz frequency out, it helps to separate the different instruments in the mix to make each one present and clear instead of the mix sounding muddy. I think that's all for now. Looking forward to your opinions!