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One of my job responsibilities is overseeing FOH for a church, and I am a big believer in mic'ing actual cabs, even if there is a cab simulator built into their FOH mixer. Line outs generally sound pretty sterile, IMO. Our solutiion is to mic an extension cab in the green room so the guitarist can turn it up as loud as he wants. Many churches use isolation boxes like
http://www.chrisfromcanada.com/guitar-amp-isolation-boxes/ in order to achieve the tone they want while controlling volume.
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...but back to your actual question.
With an amp like the Route 66 that gets most of its mojo from PI and power tube distortion, you'll lose all the "good stuff" if you take the line out from the preamp. Seems to me that stingray_65 had the right idea in tapping off the speaker (or output transformer).
If I were you, I might suggest that your friend buy a Mini MASS or a MASS Lite and be done with it, but if he wants something built-in, check out this thread as a start for schematic ideas:
http://www.el34world.com/Forum/index.php?topic=13376.0If he's
always going to have the speaker cab connected when he uses the line out, you could omit the 8R/20W resistor and SPDT from Sluckey's schematic in that thread, which would save a lot of space, but otherwise you need some kind of dummy load to load the output transformer. I've read that purely resistive loads can be hard on the amp and will yield less-than-stellar results, although I don't know much of the theory behind that assertion - maybe one of the more experienced guys around here can chime in about that (or disagree). Instead, you could make something using a reactive load like a speaker coil/magnet, or use a hybrid approach like Aiken does at
http://www.aikenamps.com/spkrload.html.