When I connect any recorder’s line input to my JV-80´s output, the record-level will be too low even if I adjust all controls to maximum. What could be wrong? Has anybody experienced a similar problem?

yes, i've encountered the same problems with my JV1080. You need to use a mixer to adjust the output level. My JV uses a booster in some of the patches, then it is (more than) loud enough, but if it doesn't you almost can't hear a thing. That is partly because it's a professional device and PA systems have a greater input sensitivity. There's not very much you can do, exept using a mixing console.