This thread could get very long- there are many variables.
You say the vox sound "dull", but an untreated room seems unlikely to be the culprit.
The reason for absortion is to avoid recording reflections- i.e.- the SOUND of the ROOM.
What pattern is the mic set to?
How far away is your mouth from the mic?
Is there a couch or bed in the room? is the room carpeted?
If you don't want the expense of treating your room, you can help matters by using a mic with a tighter pattern and staying within 6 inches of it when you sing. Dynamic mics pick up far less of the room than condensers, but some condensers have delightfully tight polar patterns that effectively reject side and rear signals (reflections in this case).