OK I have a DA 669 eco. great machine. The one problem I had since I sew mostly with #138 is this. Adler borrowed the entire hook from their 367 machine since it was a horizontal hook in the flatbed. The largest needle you can use with it is a 150. Try finding that size. normally #140. I use a titanium coated #160 for what I sew. To use it, I had to remove the bobbin case basket. I took both carbide and diamond bits and enlarged the slot the needle goes down thru. i then went over it with silica carbide flat Mitchell cord to polish it up. I now can sew with either a #160 or 180 needle.no problem. No more frayed or cut thread. Why DA did not enlarge the slot from the beginning is beyond me. The 669 silver line with trimmers has a larger slot in it to accommodate the #180 needle. If you sew with #138 thread, you need to do this.
I also think that leather made be a bit hard for this machine. for the hell of it, wet the leather and try sewing it and see if you still have tension problems
glenn