Steve
The Model Engineer (6 June, 4 July and 1 August 1997) included an article on "Modifications for a Warco 918 Lathe" which included a spindle speed reduction down to 40 rpm along with finer feeds and an extended metric thread cutting range.
The reduction was accomplished by an extra pulley block mounted on an extension of the "bracket plate" and driven from an additional small pulley machined/screwed to the front of the intermediate pulley/clutch assembly. I've used the names included in my copy of an old CT-918 parts list but a quick look at a 918/920 drive system will make things obvious.
To be honest it looks quite a time consuming project for the benefit gained and based on the fact that on one seems to have mentioned it to you, not very popular. The only reason I remember it is that it was on my long list of "things to do" and of course, it never got done. I'm afraid I haven't kept the original magazines but I do have a poor quality photocopy I could send you if you do decide to "go that route". The article also mentioned that Axminster used to sell a very similar arrangement for their 9180 series lathes.
I'm afraid I can't help with the DB10G as I haven't seen the drive system to see if it is similar. I must admit that I have looked at the similar Warco version of this lathe which is already fitted with an "American sourced" variable speed drive system that appeared to work very well at Donington. Although it seems a fair bit more expensive than the basic 920, I suspect it is a much more capable machine.
If you are cutting threads up to a shoulder why not use the old fashioned "amateur" method of turning the spindle by hand. I know it is not "cool" and would be hard work for long threads but it is much easier on the heart and has just worked well for me on a couple of ER collet chucks. I must agree with Tom however, the finish at the speeds I generate leaves a bit to be desired.
Please forgive me if this "post" is not correct but it is the first time I've tried. Not helped by me attempting to preview my ramblings which promptly seemed to vanish with a Google server error. Anyway hope this helps.
Best regards