Which amp will cause a larger increase in electric bill?

Thank you, gs 5556

The only way to correctly answer your question is to find out the RMS current of the two amplifiers at idle and compare them.    This is how you do it -> 

The wattage consumed for each will be the respective current times 120.   Divide that power by 1000 to get Kilo Watt Hours, KWH.  Then, do two things:  First, multiply the KWH number by the number of hours the amp will operate.   If you are leaving it on 24/7, then multiply the KWH by 720 (30x24) to get KWH for the monthly billing.  The second thing you do is look up the "Maximum" KWH charge on your electric bill.    Multiply the KWH number for the amplifier by that max KWH charge.  That will give you the $ charge per KWH on your electric bill for that amplifier.   Note that some utility companies charge extra if you consume too much power, so that number might bounce into the next charge bracket.  If so, then the charge will be a bit more. 

For example, assume one amp draws 100W at idle.   So, 100/1000 = 0.1 KWH.  If you leave it on 24/7, then you will use 0.1 x 720 = 72 KWH.  If your max charge on your electric bill is 30 cents per KWH, then you will be charged $21.60 to run that amplifier, $0.3 x 72KWH.  

Another way to look at it is if one amp pulls 100 W and another pulls 120W, then the 120W amp will cost you 20% more money to run it than the 100W unit.   But if you want to know what that dollar number is, you have to do the math. 

Lastly, this will be a very close approximation.   The reason is we assume the line voltage is 120VAC, which it won't always be that number.   It will vary, both up and down.   Consequently, the charge will vary a bit.   Also, that 120VAC is nominal for North America, it is different for other countries, that is 100VAC for Japan, 230VAC for Australia, etc.   Use the nominal line voltage for your country and make sure that idle current was measured at the line voltage you are going to use. 

If sitting idle (power on), which will cost more, a class A 100 watt (Threshold) vs a 250 watt A/AB (Threshold)? I have both and a few more. I'm not worried about electric bill, just wondering.

Kill A Watt

Huh... and here I am looking like the dumbass with a 2x 31wpc class A amp that's 34% efficient. Build Pass I guess....

The Kill A Watt meter is ideal for things like freezers or refrigerators.  These items vary their power consumption on whether their compressor is running, how often the door is opened, and what the ambient temperature happens to be.   For amplifiers in idle, just measure the static idle current.   Of course, this assumes you have an ammeter handy.    For the 15$ it is cheaper to get the KAW meter than to buy a decent DMM.
imhififan - Well, if the Class A Threshold is really Class A, that will consume a lot more power.   When it is on and thermally stable, you should feel a bunch of heat rising out of it as it is dissipating around 500W or so just sitting there. The Class AB amp is a lot less - I'm guessing maybe 50 W or so.