placement of power conditioner

If you have a decent PLC I don't think it matters where you place it. In the rack, on the floor, on a platform, etc. The latter is what I do with mine. Sometimes it amounts only to a piece of MDF, OR hardwoods.

To some degree I've found the surface on which one is placed can or may affect the sound some. In addition to the wood thingy's I also use compliant polymer footers underneath them for further isolation. They work on stuff which doesn't rotate too.

the only other thought is if the PLC gets hot. If so, I'd sure put something under it if on the carpeting, or throw it in the rack and find out if it'll affect things... then all you have to do is re-route power cords.

Mine is to the side of the audio rack and sits on a piece of wood. The PC sits on cones. I think it performs better on a hard surface with some isolation.

Mine is above everything else and not inside the cabinet since it runs so hot.

Many audiophile power cords have a lot more metal mass compared to stock cords. The web of connected power cords combined with the PLC chassis material can make the PLC susceptable to mechanically resonating like any other component. This is in addition to any concerns about about RFI or EMF. If you can mechanically isolate the PLC, you may hear an improvement in sound.