Audio-grade rectifying diode?

       I don't believe anyone is claiming, "audio grade", in reference to their diodes.

       However: a number of gear manufacturers offer fast/soft recovery diodes (FRED/HEXFRED, etc) as upgrades to their equipment.

       Personally (since their appearance in the marketplace): I've been upgrading my own equipment, as well as numerous others', with them and we've (invariably) enjoyed the benefit of improved presentation.

       Caveat: Schottkys can appear tempting, but- can end up noisy in some audio circuits.

       Ignore any that claim changes in rectification isn't/shouldn't/can't be  audible.        

                         ie:   https://www.partsconnexion.com/diodes-rectifiers.html

Apparently, changing IN4007 to UF4007 is beneficial. 

Bypass any rectifier diode with a 0.047uF polypropylene cap and it becomes an audio grade rectifier.

@gs5556  the amp already does that actually. But why is that so?

Don't bother with caps across diodes. Add a 0.22ųF cap in series with a 10R resistor across the power transformer secondary.