Help: 18Khz+ Amp and Speaker Impedance matching

I have a circuit that generates a range of sine-wave signal frequencies using an LTC6903 chip.
I need to drive a speaker - the small PC speaker will be fine - up to 20KHz. I don't know how to design the impedance matching circuit or the amplification portion. It would help to run it of +/- 12V.

Alternatively, with the proper impedance match again, I can input the signal to one of the inputs of a 50W per channel Sony audio amplifier (some 30 years old now).

What should I do and how should I do it?

 

I tried this on the bench with a +5V supply to the osc. chip. The open circuit output (AC coupled) is between +/-2.5V. However, when I connected a PC speaker to it (even through a unity gain 741 Op Amp at +/-12V), the signal level dropped to nothing (sharp positive and negative spikes). I felt the problem was the impedance mismatch. The chip output impedance is 45 ohms and the speaker is 8 ohms. Can you comment on this?