I believe the minimum and maximim ground phase capabilites for all X-TERRAs is the same. (except for the X-30 which was factory preset and not adjustible) The difference between the others is how "fine" each model can be tuned in between the min and max. On the X-50, the range was only from 0 - 20. On the X-505, it goes from 0 - 50. When I refer to a "magic number" for determining whether you can benefit from using a DD coil, it is based on what I found worked best on the 70 and 705. On those two models, a 28 seems to be the break point. Since the 705 has a range of 0 - 90, I use math equations to get a number for the other units. In other words, if 28 is used for my 705, 28 divided by 90 = .31111. Take that .31111 X 50 (the range on your 505) and you have a new "magic number" of 15.555. So in my previous response to you, I used 15 - 16 as the "break point" for using a DD coil. If I were using the X-50, the "break point" would be 20 X .3111, which = 6.222. So I'd use the ground phase number of 6 on the X-50, to determine when a DD might be the best choice. JMHO HH Randy