When you say "tuning" a crossbow, I assume you mean adjusting it for better performance.
With an Excalibur, you can play with the brace height a bit by putting in or taking out twists pretty easily. As long as you stay within the brace height marks on your riser, you will be fine. This adds or removes a few pounds of draw weight which can tighten up your arrow grouping some.
Also, playing around with different arrow lengths and spine can improve grouping and vibration.