Code P0300, random misfires?

So I have been getting a p0300 for some time and the engine has been hesitating more recently. Some days it runs okay but feels like the engine is running too rich causing some power loss and other days it will run super rough to where the engine will pretty much stall. I have sprayed ether around the Plenum and air filter tube and cannot find any vacuum leak up on the top of the engine. I replaced all of the spark plugs and ignition coils. I have also replaced the MAF airflow sensor. Just the other day, I got a new code p0174 saying bank to his lean. However, I used to get about 25 MPG’s and now I get about 15 MPG's, and I have black residue on the back of my car from running rich. I ran a test on the voltage of my different sensors, and it looks like bank 1 sensor 1 is bad because it’s at half the voltage of all the other three sensors. Before buying sensors and replacing them, I was curious to hear your thoughts. Thank you.