After reviewing everything it seems Minelab has provided conflicting information. I'm not sure which is correct and I'll send a request to Minelab for clarification today. Here's Page 29 of the Instruction Manual seems to indicate the 600 doesn't have 20 and 40 kHz.
No conflicting information, it's just that Minelab has done a poor job of addressing the confusion that folks have had about the frequency differences between the two machines since launch. The downside of all the hype with the Equinox is that misinformation spread just as quickly as the excitement, and Minelab to this day has an uphill battle trying to correct the misunderstandings out there.
Correct, the EQ600 does not have 20 and 40 kHz as individually selectable, single frequencies. The EQ800 does have 20 and 40 kHz each as individually selectable, single frequencies in most modes (beach mode only uses multi-freq - single frequencies cannot be selected with either the 600 or 800 series in that mode).
For example, in Park 2 on an EQ600, you can set the machine to hunt in 5 kHz, 10 kHz, 15 kHz, or Multi. With an EQ800 in Park 2, I can do the same, but I can also set the machine to hunt in 20 kHz or 40 kHz, instead, if I wish.
But when set to hunt in Multi however, both machines function the same. This comes directly from Minelab's website:
Minelab Treasure Talk: Equinox 600 vs Equinox 800
The most relevant part of the article appears a couple paragraphs down, next to the bolded
Multi-IQ, and I'll quote it here directly:
Most of the confusion I have seen online revolves around Minelab’s new Multi-IQ simultaneous multi-frequency technology and the frequencies employed by the two detectors. The EQUINOX 600 is limited to 5 kHz, 10 kHz, and 15 kHz single frequencies, while the EQUINOX 800 also offers these plus the additional higher frequencies of 20 kHz and 40 kHz. This does not mean that the 600 is not employing the full multi-frequency range as part of the Multi-IQ processing. Multi-frequency operation is identical in the two models and provides the same maximum signal response to targets in Park, Field and Beach Detect Modes., The real magic of EQUINOX is in the Multi-IQ technology (not the single frequencies), therefore you can rest assured the EQUINOX 600 matches the EQUINOX 800 in this regard.
(Note: emphasis is Minelab's)
I won't pretend to know how Minelab makes Multi work the same in both machines - but perhaps one way to wrap your head around it is to imagine that the electronics inside the EQ600 and EQ800 are essentially the same, but the user is only allowed access to 20 kHz and 40 kHz as individually selectable frequencies on the EQ800.
All that said, keep in mind that the two detectors have subtle differences that go beyond just the frequencies which can impact performance. "Gold" mode is not available on the EQ600, even though it theoretically has the identical Multi frequency functionality as the EQ800. Iron Bias and Recovery Speed selections have greater resolution on the EQ800, which will create slight performance differences between the two detectors when they are adjusted on the EQ800 in ways that the EQ600 can't quite match. For example, Recovery Speed 3 on the EQ600 equals Recovery Speed 6 on the EQ800 (see Minelab's website:
Minelab Treasure Talk: Recovery Speed & Target Masking). The EQ800 has Recovery Speed settings 7 and 8 which the EQ600 cannot achieve, theoretically giving the EQ800 an unmasking advantage in some situations. Recovery Speed 5 on the EQ800 falls between 2 and 3 on the EQ600, demonstrating the EQ800's ability to be more finely tuned to site conditions.