The clamp meter is to find the true AC amperage that your amp is producing. The volt-ohm meter it to find AC voltage.
You just do (V)*(A) to get watts (this will tell you what you amp is really putting out at a given freq.). And (A) divided by (V) to find your impedence at that frequency.
You could get a cheap clamp meter on ebay for like $25 shipped, but I'm wouldnt know about the quality. Personally, I would rather have one w/ a digital display...it would be easier to read.
On the other forums, I hear that Fluke makes a good clamp meter though. I'm looking at prolly just getting one of the cheap ebay ones pretty soon though.
I have an analog clamp meter, but I have never used in to check for true output or box rise. I have seen people use clamp meters to do this...I just havnt had to chanc to do it w/ my amp yet.
Subfanatic...what sub are yall talking about? (just wondering)