Controlling a compression driver
There are dozens of options for Class-D power amplifiers on Ebay, etc. Lots of 50W + 50W sterep power amps.  And if you are driving a 16 ohm load, you should probably use a "stereo" amplifier in "Bridge-Tied-Load" (BTL) configuration. A 16 ohm load used as BTL will present a very standard 8 ohm load to an amplifier.  Else you will need an amplifier capable of driving 40V (as you have already calculated) and that will be considerably more difficult to find.  Stereo 50+50W Class-D amplifier boards (and complete amplifiers) are readily available.

You have not revealed why you think you need to use an Arduino to control (or execute) your triangle-wave generation.  There are likely more conventional ways of doing that which may be simpler.

Note that 100W into a compression driver has the very real potential of being destructive and even fatal under some conditions. Since you have revealed nothing about your application, we must assume that you know what 100W worth of sound is going to be used for and how to avoid damage and/or casualty.