In addition to online salary resources, I also contacted a few local job recruiters (head-hunters) to see how the salary levels compared. The online resources were inflated a bit but weren't necessarily too far off the mark. It's difficult to zero in on a specific number for an MEP firm because every firm is different and will have its own pay grade structure.
When the economy tanked back in 2008, our wages were frozen for a period of 12-16 months. While I was ok with that, we certainly lost ground in keeping up with the norm for my area. To compensate, I went to my boss and asked for a 10-15% raise. I specified a range because I didn't want to be held to an absolute number. I wanted my management to make a decision as to how much I was valued. In the end I believe the raise came to something like 11%. That was in addition to our annual 3% merit increases. I eventually left that company (after 6+ years) to go work for an engineering power consulting firm where I was able to negotiate a 20% salary increase from where I was at with my previous employer. I'm actually even considering making another move soon.