PRE G2 Grounding - Why does it make such a big difference?

[Zlone]

I own a PRE G2 paired with an AMP v2, and I love these components. Recently while tweaking some things, I disconnected the separate ground on the PRE cabinet and forgot to put it back. My system was significantly lacking in joy factor for a couple weeks before I remembered to reconnect it. Once back in place, the joy returned. This is a unique requirement for this component, does anyone understand why this extra ground makes such a huge difference in sound quality?