The conversion for amps to mah is 1 AMP=1000ma. So the hangar 9 unit puts 4,8 or 16 times the load on your batteries that your current unit does.I don't know of a reason You ould need to change your volmeter.Perhaps if You are flying Giant size planes,with huge D-size packs.
I noticed that You said You are getting a DX7 radio.The transmitter uses reverse polarity on it's charge jack.The center pin is nagative like JR transmitters,NOT positive like Futaba and Hitec.If You use a voltmeter or any aftermarket chargers with the radio,You will have to buy a wiring harness for JR type or reverse the leads on you Futaba/Hitec harness. ( This applies to the Transmitter only.The receiver pack has standard polarity.)