It is pretty well assumable that a multimeter will do AC voltage, but for audio work the M-30 manual is assuming you have a True RMS voltmeter...RMS stands for Root Mean Square. AC voltage can be measured as a linear wave right? Up and down, back and forth, whatever terminology you want to use but the current is alternating across a ground or "0" reference and if you measure that over time you get a waveform...a sine wave. A non True RMS volmeter measures the peaks of that wave to give you voltage reading...it is a peaking meter, but typically when working with audio signals we want to measure the average peak energy of the sine wave, which is going to present as less than the peak right? Because of the curves in the sine wave. So if you want to calibrate the M-30 (that's just for the meters and stuff right?) then you should get an AC voltmeter (will likely be a multimeter) that is "True RMS". As long as you are doing general level calibration (typically done using a 1Khz sine wave tone) then a general duty True RMS meter will do. Might set you back $50 ~ $60 or so. If you are doing more intensive calibration like on an open reel machine then you need a True RMS meter that has a greater bandwidth than most general duty meters (i.e. is accurate at frequencies above and below 1kHz because with calibrating an open reel machine you are often called to measure signal strengths down to 60Hz or even lower and up to 10kHz or even 20kHz...most general duty meters are inaccurate much below 400Hz and/or much above 1kHz). Those meters are going to run well in excess of $100. Look at the specs in the manual for a meter you are considering if you need a True RMS meter that is suitable for the audio spectrum (i.e. 20Hz ~ 20kHz).
Another way to do this since the calibration on the M-30 is just tweaking trimmers so what goes in is what goes out is to use a VU meter on an open reel deck that you know reads "0" for a given input level. What I mean is that if you know a meter on your deck is correct and reads "0" for a -10dBv input at 1kHz, then you could attenuate the output of your tone generator so that the meter on the deck reads "0", and then pass that tone through whatever you are tweaking on the M-30...you know what is going in and then you connect the deck's meter to the output of the board and tweak the trimmer on the board so that the VU meter reads "0". Now you know what is going in level-wise is what is coming out. This will work for these more simple calibration procedures as long as you know what the meter means when it shows "0" or "+1" or whatever. Some tone generators are setup to output a tone at a fixed level which is nice. That's your known then, and it doesn't matter what that reads on the tape deck's VU...you just want to make sure that what is going in is what is coming out. That's why you are calibrating.
I haven't looked at that section in the M-30 manual so I hope the advice applies.
