High-end meter
If you want high-end metering without too much effect on sound, consider the following:
1. Choose the optimal or most convenient place to measure. This could be at output, one MOSFET source or drain, input current etc. etc. etc.
2. Choose sensing element. I would suggest current transformer on input or output stage as this is non-intrusive and measures true drive + has the bonus of providing a balanced output signal at whatever DC level you require. Other options are typically resistive. High-end sensing means an accurate .01 Ohm or so resistor with kelvin terminals. Low end sensing means standard resistor of low value.
If you need to measure current drawn from supply you basically have to measure at the power supply.
3. Choose amplification (if required which it typically would be). May I suggest cheapest op-amp available? Remember you will need some cheap power as well.
4. Choose low pass filter topology and frequency of same. May I suggest simple RC filter at 1-5Hz?
My personal favourite would be current transformer sensing of primary. You can check up on current transformers at
www.coilcraft.com or wind your own. Here is a photo of one of my favourites up to 10 amps non-contacting
www.coi1craft.com/misc/sen60p.html, and probably even better
http://www.coi1craft.com/sen60tr.cfm .007 Ohms current type at .1V per Amp ...
Petter