On almost all of the arms I am familiar with, the arm tube is parallel to the headshell, even if both are not in the same plane. Assuming that the cartridge bottom is parallel to its top, then, all three -- arm tube, headshell and cartrige would be fixed at the same angle. Therefore, if the manufacturer says to keep the cartridge parallel to the record, the easiest way to determine this is to look at the arm tube because of its much greater length. I like using a ruled 3 x 5 index card placed behind the tonearm when the arm is on the record surface in the playing position. This establishes a parallel starting point from which one can make adjustments.
If the sound is brittle, sibilant and harsh sounding, one needs to lower the arm. If it sounds too shutdown on top, muddled and dull (like a blanket has been thrown over the speaker) then raise the arm at the base. Lowering the arm may restore bass that is lost due to incorrect VTA, but, it cannot really ADD bass that is not there in the system. I personally would not use VTA adjustment as a means to increase bass -- correct VTA improves imaging, top end response, recovery of room ambience cues, etc., more than anything else.
If the sound is brittle, sibilant and harsh sounding, one needs to lower the arm. If it sounds too shutdown on top, muddled and dull (like a blanket has been thrown over the speaker) then raise the arm at the base. Lowering the arm may restore bass that is lost due to incorrect VTA, but, it cannot really ADD bass that is not there in the system. I personally would not use VTA adjustment as a means to increase bass -- correct VTA improves imaging, top end response, recovery of room ambience cues, etc., more than anything else.