I'm looking to buy a nice DMM and RMS meter, unless there are some that do both. They will be to set my amp gains and to check how much power my amps are actually putting out. What do I look for when buying these? I've heard some need to be calibrated every now and then? I had no idea. If this is true, how often does it need to be done and what else needs to be done? Also, what brands have you had good experiences with (reliability, accuracy, etc?