​Question: 

Can I Input a Line Level Signal Into DBX 286s, bypassing the Mic Preamp?

Answer: 

Yes, using a TRS Insert Cable and the Insert Jack.
Using an insert cable [ such as the Hosa STP-204 Insert Cable - 1/4 inch TRS Male to Dual 1/4 inch TS Male ] you can input an unbalanced line level signal after the mic preamp, using the Insert jack.  

1-Plug the TRS (3 circuit) plug into the Insert jack on rear of DBX 286s
2- Plug the Ring TS (2 circuit) plug into your line level source
3- Suggested to tape or insulate the remaining, unused Tip plug so it doesn't touch anything and introduce noise.