Audio Amplifier which can Supply 20W to 20-40 Ohm Load
An ordinary audio power amplifier should be able to handle your application.  For example, an amplifier rated for 100 watts into 8 ohms has to supply an output voltage of about 28 VRMS.  This voltage, with a 40 ohm load, will develop 20  watts. With a 20 ohm load, it could provide 40 watts. There should be no problem using a higher impedance load with typical audio power amplifiers. The only drawback is that you need an amplifier with a higher power rating than you need for your load.