I received a CP2000 notice regarding 401(k) loan assets. 2007 I rolled over a loan. The original trustee had sent IRS a 1099R showing the loan amount. The new administrator sent me a letter showing receipt of those funds in full. I sent these to IRS in response to show I did not take a distribution. IRS says thanks, still owe. I contacted both trutees, they refuse to send any further docs saying I don't need them.
You don't necessarily need an attorney to respond to your CP2000 notice but it wouldn't hurt from the aspect that an experienced has been there before. I suggest that you always respond to such notice via U.S. Certified Mail, Return Receipt Requested in order to ensure that they receive your responses. Lastly, the IRS cannot simply assess additional tax from a CP2000. They have to send another notice called a Notice of Deficiency before they assess the tax (assuming that you don't agree to the assessment.) When you get the Notice of Deficiency you have the right to file a Tax Court petition before the tax is assessed. It's far easier to "win" at the CP2000 level so you want to give it your best shot.