Let me join the others in welcoming you to the X-TERRA forum. And thanks for the kind words about my book. Like you, I used the White's 6000 series for lots of years. To this day, I don't think anyone ever made a better discrimination system than that used on those models. The subtle bounce of the needle on that analog meter was something you had to experiece, to appreciate. When I heard White's was discontuing the XLPro about 6 years ago, I made every effort to scarf up any new ones that dealers may have still had in stock. Fortunately, I found yet another new one about 18 months ago, and bought it. Unfortunately, it is still in the box! But I can't imagine not keeping it around as they provided me so much enjoyment over the years. Kind of like the new Advantage that I come across last year. But I regress! During the course of the past 10 years or so, I've tried about everything out there under the sun. You mention the M6....... I enjoyed using the MXT for several years. And I still think it is a good detector, for what it is. But as technology has progressed and I tried other detectors with more functionality, I found myself becoming more appreciative of notch discrimination and multiple audio tones. When the M6 came to be, I figured I could sacrifice the notch discrimination, and hoped it would be more like an MXT with multiple tones. Putting it nicely, I was disappointed in the M6. Several times!
Needless to say, I really like my X-TERRAs. The 705 is a truly remarkable detector. Multiple tones, notch discrimination, Auto NC, Auto GB, Tracking, tracking GB offset, Beach Mode, light weight, very well balanced and on and on and on.
Not to disagree, but the 505 is not the same detector as a 705, without gold. The 705 has a bit faster processor, more refined adjustments for nearly every piece of functionality, several Auto modes which are capable of making more precise adjustments that human's can do by ear, and the Prospecting mode allows for it to be a highly respected VLF gold detector. There may be better detectors dedicated to gold. But they are not VLF technology and the price tag can approach 5 - 10 times as much $$.
Based on your applications, I would suggest you consider a DD coil for the high levels of mineralization. As such, you might consider the "Gold Pack" which includes the elliptical DD instead of the 9-inch concentric as the stock coil. Although that coil may serve you well for most of your hunts, I would suggest the 6-inch DD will provide better target separation, and it is completely waterproof for your beach use. For those deeply buried relics, you may want to consider a larger coil. And with that, I'd suggest either 10.5-inch DD. They will provide the best depth of detection in highly mineralized sites. And with the DD design, the target separation will be excellent for a coil that size. With back problems, you may find the 10.5 a bit awkward for those long days in the field. I know I do. So before you spend the extra money on the 10.5 as an accesory coil, try the elliptical that comes stock in the Gold Pack and see if it doesn't provide all the depth you need.
I would agree that typically, less weight usually meant less durability. But that was the days before high tensile plastics and carbon fiber composites. I would also say that the technology implemented in the VFLEX X-TERRA is far advanced over the boards used in your 6000. So although the X-TERRA isn't the detector in a big black box you've been using, rest assured it is plenty durable for the types of hunting you will be doing. And frankly, you will be ready to call it a day long before your X-TERRA is. One thing I've done to mine is change to a straight rod assembly. And if you're considering one of the larger than stock coils, it does help maintain a better balance. I use the straight rod assembly out of a Sovergn GT. But I've seen guys who have used a Tesoro straight rod, or a Whites straight rod assembly. Depending on the make and model you get, it may be as simple as redrilling the holes where the new rod fits into the Minelab upper rod. (Minelab uses one tab on the bottom of the rod and most others use 2 tabs, horizontal) But that is an easy fix with a drill and a bit of patience. The environmental cover by Minelab is a tight fit. Although battery life is excellent (+/- 40 hours per 4 cell set), it is a pain to take off and put back on, when you are in the middle of a hunt. There is a company in Europe who makes a cover that has a velcro "lid" for the battery compartment. They do not support this forum, so we are restricted from posting the site. But if you do a search online or on ebay, you will likely find what it is I am talking about.
Again,welcome to the forum. If you have any questions, rest assured that there are a LOT of very knowledgeable folks here to help you with the answers. HH Randy