USB Gen 5 in my Gumby came back today. Been warming for about 4 hours.
It's hard to say there is a difference, since subbed my Bimby into the chain in place of the Gumby while it was out and so my ears have tuned to that setup. What I can say is that the difference between the Bimby ( with USB Regen) and Gumby (direct) used to be subtle. That is no longer the case. The later is clearly more natural and revealing now. I don't know all the terms to describe it, but I'm glad I made the upgrade ( though still annoyed I sent it in.) I'd recommend if your amp is clean enough pass the changes through.
Gumby > Bottlehead Mainline > HD800