The 6000 line from the 70's and 80's has quite a following. Once I discovered that it had a low speed gearbox in it I had to get one for myself. I have around $350 into it (after repairs) but they seem to hold that price in the market. I had to have it repaired because of the extent of the problems. It had the standard issue "locked up" control knobs; if these machines sat around (which most all of them have) the knobs will seize up solid. These are not easy or cheap to fix.
It is an interesting machine, much newer than I'm used too. My interest in the low speed gear is not the punching power but the ability to follow curves on my leather shoe parts. It is not a quiet machine either and in the low speed mode it makes a real racket. It also has electronic needle position stop pretty fancy for back then.
OK, the ability to go really slow with excellent control is what I was hoping for... and i got it. The low speed is exactly what I have been needing; it takes hours to make ready each part of a shoe, the last thing you do is sew. I am going through an 1/8 in (3mm) of leather with better control than ever before. So far I am very happy
After 5 years I'm back to the 6570, I'm going to see if it can replace the Nora (I doubt it) I have adopted using decorative stitching inside of my shoes and would like more options outside of ZZ.
It is one of the easiest machines to thread and aside from the unappealing mechanical noise (unlike the Necchi) it is a sweet stitcher.