Thank you!

Simply place an inactive vent cover on the other side if you want the symmetry.

Toekick would be better functionally, pushing air into the room instead of cooking your shelf. Also the depth of the space under the shelving is fairly deep when open.

@3onthetree - Thanks for your comment! I worry that the toe kick vent is just unattractive. It will be very visible, unlike a toe kick vent under a kitchen cabinet. This is right in your face as you look at the fireplace, so symmetrical toe kick vent or not - it sounds unattractive. But, can you explain what you meant when you said the "depth of the space under the shelving is fairly deep when open?

I assume the existing cabinets are 12" deep. In the picture, the "legs" really get lost visually, they do not look like a piece of furniture because it is flush against the fireplace surround and wall, and of the same color. So the shadowed depth underneath looks too deep and unnatural. If it were an independent piece of furniture, then the depth of space underneath is expected.

I'm not sure how deep the new cabinets will be if set back 2" from the face of fireplace (like 6" shelf?). I would look at a linear slot diffuser in a toekick.

The cabinets are 12" deep. I think I feel better about deep space than toe kick vent. But, I don't really know what a linear slot diffuser looks like. I asked AI, but didn't get a good example of one in a cabinet. Do you have an image of one?Again, in a kitchen cabinet, the toe kick vent is less obvious - I will have one in my kitchen remodel, and I am fine with that. But, these fireplace built-ins are right in the sight-line.

Adding a vent deflector like this will help redirect the air from the vent into the room instead of blowing up into the bottom of your cabinet if you decide to stick with cabinets with legs.

AI forgot to google: