DLP is still a projection technology and is FAR less bright than plasma, even moreso compared to LCD. They really should be used in rooms with minimal ambient light, and certainly no sunlight, for best results.
That said, if you've owned the unit for a number of years, then perhaps it is time to replace the lamp. The lamp in a projector will fade over time and may lose as much as 75% of the original brightness that it started with. So, if it has 4,000+ hours of use, you may find that a new lamp restores the missing brightness which you originally had with the setup.
If it just isn't enough, then the Sharp 70" or 80" LCDs are a good way to go.