I can't figure out how to adjust the bass on the JVC KD-HDR30.
Like with many other JVC receivers, adjusting the bass is done from the "EQ" (equalizer) mode for one or all of 6 presets: ROCK, CLASSIC, JAZZ, HIP HOP, POPS, and USER.
To adjust it, select the desired preset mode, use the left/right rocker (on volume control) to scroll to the desired frequency, 60, 150, 400, 1K, 2.4K, 6K, and 15K, and then turn the volume control up/down to set a level from -5 to +5. 60 and 150 adjusts the bass, 400, 1K, and 2.4K adjusts the midrange, and 6K and 15K adjusts the treble. Then you select whichever EQ setting sounds best with the particular music you're playing.
Hope this helps.