My income is not low - it is just a lot lower than the guys around me. There is no possibility for me to talk to another firm and use that offer as a negotiation tool. I would be shown the door immediately if I did that, and which will probably be followed by a letter reminding me about my obligations etc.
By the way, I used to switch a job every 3-5 years and get a decent bump in pay every time. This is the one job where I felt I could not easily find something better.