What volume level are you monitoring your work?
Problem rooms do not improve with the application of "louder".
Any well recorded commercial releases which are similar material to what's being recorded will show frequency issues if the culprit is the room rather than particular instrument eq miscalculations.
Don't spend money to fix what can be corrected other ways. Make certain the culprit is the room. Unless you are doing movie soundtrack mixes, don't use a sub speaker/speakers. Use the right size monitor speakers for the distance you are from them.
Monitor at the lowest volume that get great results. Be advised that I can't stress this enough.
there's more to consider but there isn't enough info for definitive answers. One person's "boom" can be another person's too much bass and vice versa.
Having a better idea of the actual frequency range of the issue is pretty key to getting the better advice.
The longer you monitor at higher than necessary volumes can "flatten" your hearing response and the danger is inching louder and louder looking for better. The result is generally worse and worse in actuality.Turning the overall volume of what is being monitored will NOT resolve a room related issue.
Try to determine if the "boom" changes depending on the key of the song being recorded.
Keep in mind that a room need not be "perfect". I can't tell you how many supposed "acoustically perfect" venues quickly sound horrid with certain material played, or volume levels that the room simply won't support.
A great room is one where the mix is good without regard to where recorded material take elsewhere to be listened to. The best places to record and mix allow whats done to easily translate in other monitoring situations.
Perhaps using some saturation in the form of a plugin might go a long way to better audibility for bass guitar parts. Maybe see if this freebie plug might be of use: http://www.softube.com/index.php?id=satknob It's not a magic bullet, but some material benefits from some harmonic complexity
assistance on bass guitar.