If you hoist a GPX 5000 for any period of time you realize just how stunted VLF detectors are when it comes to raw power / depth of detection. The only problem is the discrimination is limited, but more and more people are learning to use them effectively. Just like the old non-discriminating BFO and TR detectors you can learn a lot about targets through use. And the GOX does have tones and iron blanking. They are different and limited but they do tell you things once you get over it and work with the machine.
The power is such that even at outrageous prices more and more people are turning to PI detectors and dealing with the junk. The reason is simple. Eventually any location will only give up so much to any and all VLFs. If you want to keep going, get a PI, and be amazed at what was left behind. But you will work for it!
I am far more interested in seeing more done to bring more and better PI technology to market. Less expensive, more compact units. Better discrimination is the main thing. I would pony up $10,000 tomorrow for a GPX with CTX discrimination.
VLF, whether single frequency or multi-frequency, is pretty much tapped out. You can only put the same thing together so many ways. I honestly have not seen any real improvement in VLF since 1990. A Fisher CZ-5 still runs with the best made today.
My advice - keep an eye on PI.