The guys already touched on the placement issues... Just to review:
Front baffle should be in front of the edge of the stand or cabinet.
Ports need room to breathe behind.
Speaker should be elevated/angled in a way that puts the tweeter within ±15º axis of you ears. I prefer to aim at shoulder level at the LP just so you aren't in the direct beam if there is any brightness in that tweeter.
When I used a horizontal center, I used these:
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They show as currently unavailable, but it gives you an idea... I turned them backwards so the speaker back was nestled against the foam ridge. Angle can be adjusted by the wedge. 1 pair for the center speaker.