"Jakeman" is right when he states that it all depends on your setup and room acoustics. My method is pretty similar in that I have four subs always start off with one sub, set the volume and then individually do the others to match the first in output level. A good SPL meter AND "test disc" is worth the expense here since low bass is harder to balance strictly by ear AND, in my opinion, in comparison to your AVR/Pre-Pros internal test tones, the sound coming from a source piece of equipment like a DVD/BR player gives you a better (and more accurate) idea of what you are going to hear when playing back source material. You then turn them all on and match the volume to "taste" or, if you wish, use the meter to match them with your other speakers.
If your subs are equidistant from your listening position, the distance probably won't be a factor UNLESS, the subs are from different manufacturers and/or different types(down-firing/front-firing). At this point phase "may" have to be adjusted and this could affect overall volume as well. It may take some time but once it's done, it's done. This is, of course, unless you decide to move or replace them with others.
In the end, to me anyway, the TWO most important elements of maximizing sub performance and from the standpoint of your listening position, is balanced volume levels and proper phase relationship. How loud you want to play them in relation to your other speakers and source material, is just a matter of personal taste.
