The 400 watts listed would be the power it draws from the wall voltage to supply the amp. The actual output power is listed as 60 watts RMS.
Personally I don't like Sunn amps for guitar, and love them for bass, but YMMV. They are clean up to a point, and then overdrive quite hard as compared to an old Fender. A lot of heavy metal guys would probably like them for that reason. They can be modded to work better for guitar but they sound fine stock too. If you get a fuse for it, you want a slo-blo fuse and not a fast-blo. I would also suggest that you turn the amp on after replacing the fuse with all the tubes out. If it works fine there, then turn it off, put the rectifier tube in (the GZ34) and turn it on again. If it works fine without blowing a fuse there, then add the other tubes in and try again. The fuse usually blows because something caused it to blow and it could be bad power tubes, or a bad power transformer, or other things like that.
Greg