Earphones driven by preamp RCAs?

The jack ain't the issue; it's what's driving the jack. You preamp has a relatively low output impedance and does a fair job of driving the phones. However, the fact that you have to set the VC at 12 o'clock suggests that there's not much power output available.

A dedicated phone stage, regardless of the output jack, will have more power and a lower impedance. The result will probably be lower distortion.

Thks,Kr; I kinda knew it had to be something like that.

I believe matching the pre output and headphone input impedances will maximize the power transfer. So, anything else different between a pre with headphone jacks and one without?

I am also interested in finding out more on the topic....lately I find myself browsing through the for-sale ads for headphones, but my current setup (pre, cdp or amp) does not have a headphone jack. It would be great if I could avoid buying a separate HD amp. I'll be keeping an eye on this thread...

It may maximise the power transfer but if there isn't enought drive power to begin with it won't matter.