Connecting iMac to stereo receiver

That will depend greatly on the relative quality of the DAC & associated circuitry in whatever USB audio interface you might use vs. the quality of what is in the receiver.

In general, a high end external audio interface, especially one aimed at the audiophile or professional markets, will outperform the converter in a typical receiver, but beyond that any more specific advice is impossible without knowing more about your home stereo & which USB devices you are considering using.

I do not agree. The headphone amp outputs a conventional two channel analog signal on the multi-purpose 3.5 mm audio output jack on all Intel iMacs. But that same jack outputs a digital optical signal if the iMac senses a mini-Toslink adaptor plug has been inserted instead of an analog stereo TRS (tip ring sleeve) plug.

IOW, the same jack furnishes two entirely different kinds of output (analog or digital) depending on what is plugged into it. The digital output encodes the audio (which could consist of multiple channels of audio) on a single beam of digitally modulated light. The analog output is two separate signals, each on its own electrical conductor, derived from the internal DAC in the iMac & amplified by the amp to provide enough power to drive headphones.

It is possible that the amp does double duty to also drive the light source for the digital output, but that is in no way is comparable to using it to drive headphones -- for the digital output all it would have to do is switch the light level between the digital high & low states.

My guess is that whoever told you about the headphone amp either did not know about the dual function of the jack or thought you were talking about an analog connection to your stereo.

I am currently sending an analog signal from My (Late 2006 model) iMac to a Pinoeer receiver. Headphone jack to RCA (LR). The sound out of the receiver switches to a single speaker after a few minutes and then just flickers back to two speakers intermittently. Any other device connected to that receiver (even via the same headphone jack) does not seem to have that issue. Should I be using a specific cable or invest in a new receiver?