Unfortunately, simply listening without an accurate reference point is all but useless. I recommend a very high quality and linear headphone as a reference point. This reference point is unaffected by room acoustics and it will provide a steady reference in which you can make notes to compare. However, you must try to equal SPL between the headphones and speakers as well as possible. It can be done measurably with a SPL meter, but it will still have a 1dB or slightly higher error. To equalize volume - use a *band limited white noise source(500hz-3khz band is sufficient) and set the speaker volume at listening position to a predetermined Db level. Now, to set the headphones, play the same signal through them, but have a small wooden box made with 4 sides(no front or back is needed) that is the same width of your head, with a hole drilled in it for the SPL meter mic to fit into(have the mic fit flush with board surface and use some thin closed cell weather stripping material to line the hole for a snug air-tight fit) at the ear position and a shelf for the SPL meter to rest/be stabilized. Place the headphones on this and play the signal and adjust volume to achieve the desired SPL value.
You can also set volume by ear, but a 2 or 3dB error can easily result.
-Chris
*Band-limited signal is suggested in order to avoid the frequency response differences in response that will occur outside of the suggested band due to head/jig differences.