Stacking gear no more rack space does it affect sound quality

     Power supplies and electronic circuitry, of a necessity, generate RFI/EMI.    That such can interfere with the performance of adjacent electronics; SHOULD be the most basic of one's understanding.    Particularly, if one assumes the position of (imagines themselves) an advice giver!

      Digital gear is especially bad, when it comes to such emissions.

      I have three pieces of my equipment in a vertical cabinet.   The topmost are my TacT preamp and BAT CDP digital noise generators (SS Bass amp and it's massive power supply, below).   On the cabinet's top: my turntable and of course: highly sensitive cartridge and tonearm wiring.

       Those pieces are all separated (the shelves and top completely covered) by Texas Instruments RFI/EMI blocker sheeting.    Something Michael Percy used to sell, but: I believe has since been discontinued.  

        TI does still offer things to mollify the effects, as do a number of others.

         Not everyone will bother with attempting to avoid such interference and: IF the distance between pieces of equipment and wiring can be maximized, it's not as much trouble as the effects drop off, in accordance with the Inverse Square Law.

   https://www.prosoundtraining.com/2010/03/15/the-emi-project-part-1-introduction/

   https://www.sciencedirect.com/topics/engineering/electromagnetic-interference.

   https://www.ti.com/lit/an/snla016b/snla016b.pdf?ts=1625397787187&ref_url=https%253A%252F%252Fwww... 

   https://www.daenotes.com/electronics/microwave-radar/inverse-square-law#sthash.Avvckp3b.dpbs

          That's all some of us silly musical equipment owners have the audacity to call:
                                                              SCIENCE!

If you don’t hear any noise at the listening position when everything is turned on with no music playing and the volume knob at the usual position, then you’re ok.

     ps: That TI Shield is explained on page 14, of this catalog:  

                     http://www.percyaudio.com/Catalog.pdf

     A side note: many of us have found that background noise is often inaudible and goes unnoticed, until it's gone and a, "blacker background" is experienced.

                As with everything else in this hobby: YMMV!

                                    Enjoy the journey!

It may or may not affect the sound, but I would worry more about making sure everything has sufficient ventilation for proper heat dissipation. You wouldn't think digital components generate much heat, but I recall my Oppo 205 running quite warm and I would hesitate to stack anything on top. 

Stacking is very bad for sound get a bigger rack asap.