I'm curious what problems you had with the bobbin winding - I've noticed with my Elna 860 (Janome 9900 twin) if the bobbin doesn't wind well (fatter on bottom than top), I get those same tension issues where I need to crank up the top tension to get a decent stitch (and the back looks just like yours if I don't). I haven't figured out what I did wrong in the bobbin winding - if I wind a new one it typically looks much better as does the stitch tension.
I've also found both machines prefer Aurifil (piecing and quilting) and Isacord (quilting) threads to another brand I've tried.
Michelle