Currently Being ModeratedOct 9, 2011 7:30 AM (in response to VinnySem)Re: Non 4CM (fx loop only) hookup for HD500
Your setup sounds good to me. The downside is that you don't get to use the front-of-amp effects the way you would want em. Also, you cannot easily get to the amp models if you want to use them alongside the real carvin pre-amp.
You may be thoroughly sick of the whole 4CM thing by now but it you want a final punt, I had a huge hum issue too and fixed it by breaking the ground connection in cables 3-4 of the link (the ones going to and from the amp loop). It sounds like disconnecting the second cable in the 4CM (HD FX Send to amp input) you got rid of your hum - mine was the same.
Details are here http://line6.com/support/message/317687#317687 and there are pics on jvmforum.com if you wanted to try building a ground lift box.
I also had good results with a Behringer Hum Destroyer but was not certain that I was not getting tone suck with that. The ground loop disconnect seems very neutral to my ears.