Hi there,
Apologies for never following up on this. Here's a summary of my results:
LEAST SUCCESSFUL: Connect Amp Out on Pod Go to Input on Spider mkii. Create a "clear" patch on the mkii without any effects or amp settings. I couldn't get this to work, I kept encountering a "doubled effects" type sound, which I couldn't work with. There might be a better way to do this, I just couldn't get it to work.
SOLIDLY SUCCESSFUL: Connect Amp Out on Pod Go to Aux In on mkii using a 1/4-1/8 cable. I was able to get a good tone out of the amp using this method.
MOST SUCCESSFUL: Connect Main Outs (L&R) on Pod Go using two 1/4 cables, connect to 2 1/4 to 1/8 converter (basically a y-split) to Aux In on mkii. This produced the best sound overall.
Would be interested to hear other approaches. Still strange that Line6 doesn't add a "bypass" switch on their amps in case someone wants to use a Pod Go with a Line6 amp!