I have a VSAT - I know what you mean about the volume being too loud. I found a simple solution that involves adding a few diodes to the circuit to create a "dump" for the signal when it gets too strong.

If you are not into soldering electronic components, I recommend getting a good pair of headphones with "volume limiter" circuit. Most of the quality brands made for metal detecting have something like that to offer: RatPhones, Gray Ghost - even the Jimmy Sierra brand.

I have the new MSA model from RatPhones & like them a lot. They are slim-fitting & comfortable, and have thin, post-type volume control knobs that are easy to adjust, but won't shift when brushed or bumped, keeping the volume level set where you want it. They have a full-time volume limiter circuit that works great - no loss of weak signals. It only cuts down on the painful sounds, bringing them to a tolerable level. They aren't the most expensive phones, but aren't cheap either. Food for thought...

Regarding the GMZ, it is a simple, no-frills, dig-everything machine (i.e. no iron ID). I've heard they are very sensitive. But the functionality is far different than the VSAT. It is a "silent search" machine, meaning you have no threshold hum as a baseline. It is quiet until you get a tone from a target. I suspect the VSAT & GMZ would give you comparable results in the field once you got used to operating the units. Much of detecting is a function of the operator's skills & capabilities.

If you are using the stock "Gold Master" coil with the red label & white lettering, you are using a concentric coil. That's what I have. It is more sensitive to ground mineralization, thus limiting the amount of gain you can run on the machine. All of the GMs that came after the VSAT have the double-D coil, which has the gold label & black lettering. I literally just got one for my VSAT this week. I'll let you know how it performs once I can get it over some dirt. I'm itchin' to try it out!

Gain: this control sets the power level of the transmitted signal. Higher gain = more power & a deeper-penetrating signal. However, if it is too high, you will get signal "reflected" back to the receiver coil, resulting in anything from a chattery-sounding threshold (unstable) to false signals, or even the squeal of an overload (way too high on the gain). I have posted more on the basics of gain, ground balance & the VSAT control in another thread on this board (under metal detecting). Check it out - there might be some info buried within that might be of use. Hope that helps... :-)