I spoke with the guy who had previously fixed that older amp, and he said it's basically a matter of incompatibility between the old and new technologies. He said that the Nashville 400 loop is not a true effects loop, so white noise would be expected. He also mentioned a possible impedance or gain issue. Thankfully neither the amp nor the Eclipse is faulty, they just can't play together nicely.
The recording engineer later told me that the hiss he heard in his board was due to the lack of a pre-amp.
Thanks for you help.