Which enlarger are you using? I wouldn't start swapping bulbs without a lot of research. As Donald said, the bulb is matched to the housing and power supply, so any changes may be a problem. The Omega 4x5 enlarger I use for all of my enlarging comes with a 260 watt bulb. It is possible that 100 watts is correct for a MF enlarger and a higher bulb rating would be needed for an enlarger of greater capacity.
The question of negative density is also a good one. Are you exposing with a lot of highlight detail in this long exposure print, or is it a "normal" shot?