Shooting in low-light environments

[tlowe773]

Thank you in advance for your insight...

I am a graphic designer with basic photography knowledge. My job is asking more and more that I shoot my own photography for cost saving purposes. I have a Canon 30D with a 28-135mm 3.5-5.6 lens and a 55mm 1.8 lens. Both have met my needs thus far but I am having one particular issue:

I shoot a lot of events in low light environments. Due to the nature of the events (i.e. wealthy donors milling about that don't want to be disturbed), I don't want to use a flash. I usually shoot at a low aperture as to let in more light. My images are not crisp and clean, they have a soft edge and it get worse as I color correct (see image samples). Is there another lens I can purchase that would help alleviate this issue without having to use a flash?

Any help you have to offer would be wonderful!