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There must be something wrong either with your ramps (I suppose you have it along with arduino mega) or more probably the drivers.
Or you measure the Vref voltage on wrong places. Place one probe in the middle of trimpot and the other one on Gnd pin of the driver.

Popolu driver with DRV8825 is typicaly made in such way, it prepares its own voltage for Vref from motor Vmx. The up most V3P3out it can give is 3.4V/0.1mA (typicaly 3.3V) and maximal Vref you can return back to the driver and the driver can survive (when you don't use V3P3out) is 4V.

So 1.6V in the middle of the scale of trimpot is perfectly OK, but you shall never measure more than 3.3V in the maximal position. And this can be only measured at the moment when 12V (which is your Vmx) is connected to the ramps because DRV8825 don't use 5V comming from Arduino as it have Fault pin instead of Vdd (compared to A4899, which relies on 5V Vdd from Arduino).

Now, current for motor Ifs is calulated as: Ifs= Vref/(5xRsense) where Rsense are two small SMD resistors next to the tripot. Typical Chinese drivers have Rsense 0.1OHM, whitch is nonsense, because than the maximal current for one coil can be up to 6.6A!!! and that is crazy. Even preset 1.6V means 3.3A per coil, way above the specified 2.5A this driver can temporaly give. So the driver is probably in some error state and resets it self frequently.

Try to turn the trimpot as much to the minimum as possible (beware, these trimpots can go 360°, so you can get to place where it is off and than straight to maximum). Plug 12V (without the USB cable) and measure the Vref (BTW the ramps in default setup will power up the arduino instead of USB so it will prepare 5V anyway). Try to get close to the 0.08V on X, Y and E axis and 0.16V on Z. It is good idea to use screw driver with ceramic tip or atleast with insulated handle. Turn it, than remove screw driver and measure, Repeat it until you will get there. Don't measure with the screw driver on trimpot as it can get some parasitic capacity.

Hopefully the drivers and motors are not gone yet.