I like to burn in stepups at line level, not using the iRIAA filter. With the phono disconnected! I run over night on a FryKleaner, then short the outputs of the two channels together using a female to female RCA adapter (from FryBaby), and burn another night or two.
Most stepups I know of can handle the line level signal. Check the specs of yours, to see if they do. Otherwise, the iRIAA filter is needed.
jh