I am using an RIIA Hagerman inverse filter to break in a new phono stage with a signal from my DIrectStream DAC. It may just be my overactive imagination, but it seems like the DAC sound quality is improved when routed through the phono stage (Sutherland Duo). Two questions: is there a technical reason why SQ might improve with this routing, and, is there a problem with leaving this configuration in place permanently (aside from needing to switch cables when moving from DAC to phono source)? Seems like a bad idea, but I’m not sure why.
The phono stage uses RIAA equalization which boost the bass and reduces the treble. It is completely wrong for a flat signal from a high level source.
Sorry - typo. It’s an iRIAA2 inverse filter
No reason not to leave it like that if you like it - not seen an inverse riaa filter for a while 
It’s kinda a kindred spirit opposite of the nuwave phono converter…
PS it’s probably the slight extra harmonic distortion going through the phono preamp that makes it sound sweeter 