not specifically about professional services firms, but potentially relevant:

"Ask HN: How to handle acquisition offer from a competitor?", 2012, https://news.ycombinator.com/item?id=3624505

"Ask HN: How do you deal with potential acquirers?", 2014, https://news.ycombinator.com/item?id=7074264

"Don't Talk to Corp Dev", 2015, http://paulgraham.com/corpdev.html

"Ask HN: What happened after acquisition?", 2016, https://news.ycombinator.com/item?id=12226915