Why use ground wire from preamp to amp???

The fact that you do not hear a hum does not mean that there is no voltage differential between the different devices. Star grounding eliminates in the application of amplifier surgery on preamp level.

This comes from my comment recently on a thread about hum:
Every piece of equipment has at least two ground paths between them; the signal (RCA, XLR, not optical) cables and the power cords, often the speaker cables add another dimension. So star grounding is imperative (to eliminate voltage differences and hence current flow between the grounds of electric connecting devices). You can spend an huge amount on an ultra-high end box like the Troy Tripoint or try a DIY solution a la Audience adept Response or simply a brushed (yes it makes a difference) copper rod with 10ga copper wires to the units. 

The interconnect RCA are mostly only grounded on the source side as the manufacturers know there may be a ground differential so they use use this workaround for, again from the ground differential point of view, possible deficient systems. 

/jea48: you're absolutely right. I must have a brain fart then, I meant the shield for an interconnect.

Anyway, star grounding is recommended to avoid hum.

Norm, what about the Audience adeptResponse power conditioners with ground bars doing a similar job that the Tripoint Troy Signature do?

Cheater plugs have the inherent danger if something in the unlikely event mishaps in the device to the metal housing, the user touching the surface will act like a grounding wire which may be a shocking experience if he/she lives to tell.

+1

Star grounding devices like Troy Tripoint  Signature (Norm you own or at least owned one) and in lesser extent Granite Audio Ground Zero do contribute with impedance ground matching. That brings us back to OP's question about ground loops.

Or is it the fact that the absorption of EMI and RFI within the Tripoint Signature (the same rationale goes for SR Powercell) that is the main contributor to improved sound?

Both the SR Powercell  and Troy Signature are known to be black boxes, no nudie of their models (pun not intended) on the web available, one contains the "trickled down Powercell" technology and the other some passive filters and magnets (HFC is pulling hard, pun intended, in this direction). The Troy Master Ref adds at least a AC waveform corrector so part of the 60s technology is added (BTW I have a vintage Elgar 6006B - usually under 1k on Ebay- that acts as isolation transformer and AC waveform corrector for that for my upstream equipment. 

I never had any of them in neither of my systems so would like to hear Norm's comment on the benefits of star grounding vs EMI/RFI rejection/absorption.