Earphones driven by preamp RCAs?

Hard to answer as I've never used one. However, ARs tube units have more than decent line driving.

My advice stands: Try a good headphone amp.

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.

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

1. In general, a well constructed purpose-designed device, like a headphone amp, will be superior, for driving headphones. Note, they have a much lower input impedance than that of the average amp. Of course, such a general statement is irrelevant when you get to real-world specifics.

2. Just because it's loud enough does not mean that it has enough power to work into that load without distortion. A 12 o'clock setting is pretty high (in general and without knowledge of your preamp or the gain curve of the pot). Is that where you set it to feed the amp/speakers?

3. Why not just try a decent headphone amp and see for yourself if it is better? Surely, there are web dealers with a useful return policy.