My Yamaha clips at 3/4 volume

The amp may have a device to detect problems and turns itself off before damage can occur.
You preamp is probably putting out more signal at that three/fourths volume than the maximum the amp can take and stay turned on.
Either you need a bigger amp, or you need to expect less volume from your equipment.
The amount of signal your preamp puts out just exceeds the amount your amp can accept before shutting down to protect itself.
If you want more power/loudness from your setup, you need a bigger amp.
If this is a new development, then something is wrong with the amp. Otherwise I think it is just too small an amp for yur demands.

The fact that it clips at 3/4 volume has nothing to do with how much power is being used.

Western,
Something doesn't add up here. Your Yamha M45 amp is rated at 180 Watts rms @ 4 ohms, into 4 ohm speakers that are efficient enough to have been successfully driven by a 10-watt rms Trends Audio T-amp! (Dingus' 2nd Agon system, Bedroom bliss with the T-Amp).

You should have way more than sufficient power to drive these speaker loads.
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Give us more details, and I'm confident one of our amazing A-gon Gurus will surely be able to help you in isolating the problem.
e.g.:
When you mention 3/4 volume, are you a) referring to the volume pot on the pre-amp, or to b)the peak power meters on the amp? If a), what peak power readings are you getting on the amp w/o failing?

Is your "Auto Class A Switch" deactivated? If not, try switching off to run in full A-B mode. (This could possibly effect handshake with pre-amp?)

What sources are you using?
Was this system ever more responsive?
What last changed?
What other components do you have to swap out for testing?

Good luck!

I'm using a Technics SL-PD867 CD player along with the amp and pre-amp. I had the volume pot on the preamp at about 2 o'clock, the left and right output levels on the amp were at 12 o'clock. I just got the M-45 and Aragon 28k, previous to this setup I was using a Yamaha A-700. I'll get it back to make a comparison.
When I checked to see if the Class A switch was activated, it wasn't. What I discovered was that both the A and B switches were both activated and the speakers are only connected to A. Could this have an effect?

Thank you Elizabeth, Rrog and Sandstone for your help.
As you can probably tell I'm somewhat of a novice when it comes to stereo equipment.

Based on your description and the fact that your preamp volume control is at 2 o'clock, I would set the M45 amp controls to fully open, and then listen again. All other posts above is great information as well.