I am lucky enough to have a fully functional Hafler DH200 powering my monitors.  Plenty of power, excellent response, no noise.  Normally this amp stays on during sessions; however, when mic'ing vocals or acoustic guitar, I've been turning the amp off and going into headphones until that recording is done.  Will turning the amp on and off eventually damage the amp?  Or am I ok?
Thanks.

The more serious issue is to make sure the monitors are powered down before you turn off the amp. On and off rapidly isn't healthy for an amp. You're very likely not doing that.

Turn the input levels down before powering the amp on and off.
 
My studio monitors have amps in them and they come on and off quietly.
 
As long as you are not hearing a thump in the speakers, you are good to go. No harm done.
 
Switching the amp on and off will shorten the life of the switch. It's generally rated for a certain numbers of cycles but that is average and will vary. As far as the electronics inside..... they are built to handle that on off cycling in normal duty.  It doesn't hurt to leave them on for several hours with out problems.... obviously you should shut them down when you are not using them for long periods of time..... over night would be a good time to shut them down. Also if you're doing something that doesn't need the power amps on..... shut them off. No harm, no foul.
 
If they are on, they are spinning the electric meter and running up your power bill. That's the downside to letting them run. They also create a bit of internal heat. And heat shortens the life of the components. An amp that is in quiescent mode will not create much heat, certainly not like one powering sub woofers in a loud concert..... Another danger is that electrical spikes can get in when it's setting there running.
 
It's something I would simply use some common sense about.
 
 

Make sure you turn the volume down before turning off the power, to avoid damage to your speakers. But as far as the amp itself is concerned, the most likely damage will be to the power switch itself. Now if you were cycling it off and on dozens or hundreds of times a day, that'd be another story - vacuum tubes don't like it when you do that.

Agree with all 3 posts above mine 100%. One thing you may have going against you, as much as I love my Haflers, they don't seem to last more than 2 years without going back to be repaired. AND...not many repair shops want to work on them so you guessed it...back to the factory they go...and man do they jack your jaws!
 
Most times, an amp at "idle" is not going to present any problems. Frying amps usually comes from 4 possible things:
 
(there are more, but these are (IMHO) what I'd consider to be the most common)
 
1. Excessive volume that the amp may not be able to handle on a constant basis
 
2. Improper ohmage
 
(nothing will fry an amp faster than speakers that are the wrong ohms. OK, wait, one other thing will...plugging what should be in the input into the speaker output of the amp and what would go to an output, into an input. I did that one time by accident years ago. I was talking to a client on the phone and wasn't paying attention. Lesson learned!)
 
3. Turning the amp off and on without turning down the volume on the amp
 
4. The wrong cables. Using the wrong cables can make a power amp run hotter. Like a car engine, the hotter it runs, the more you lose performance and tax the amp....and then it can fry on you.
 
-Danny

Thermal cycles cause chip creep.
 
Heat ages components.
 
My gear comes up once a day, and if it doesn't go down once a day, I usually find myself wishing it had.

Usual rule of thumb is volume down on anything before turning on the amp, and turning down all the volumes before turning it off.
 

Given I have a MOTU 828mkII I had routing issues which meant i couldn't record without turning off my monitors, so instead i went with this..
 
http://www.tcelectronic.com/level-pilot/
 
 
Worked like a charm.. 

opposite experience from Danny I'm afraid, I have a DH-220 in the studio, driving UREI 809s or Yorkville YSM-1s, and a DH-200 in the living room driving a pair of Polk Model 10s. As you can see from the model numbers, all this gear is pretty old... and the amplifiers have never caused me a problem.
 
I do turn the studio amplifiers on at the beginning of a session, and off at the end. If I need to mute them I turn down the output that is driving them. So far that's been enough to keep them all alive.
 
FWIW, the others include a Crown D-60, D-150, and PS-400, and a QSC, can't recall the model number at the moment. All fine amplifiers, I prefer the QSC driving the Yorkvilles, and the Haffler driving the 809s. But that can change from time to time, I am so fickle<G>!