No PHONO IN Solution

You need a real phono preamp. Any phono preamplifier can be plugged into "line in" inputs. The rm-4 is apparently a pre-pre phono; it only brings the signal level from a moving coil cartridge up to the level that it can be used by a phono preamp. You should say what cartridge you're using for a more explicit answer. If it is a moving magnet type cartridge, you would not need the rm-4 at all.

Thanks for the response! I'm using Blue Point V2 that comes with Pro-ject RPM 5 turntable preinstalled sitting on their Pro-ject carbon fiber tonearm. It is MC cartridge. But it outputs healthy 0.25 mV. Its output signal/voltage is similar to MM cartridges. I think I don't have to use RM-4 at all. But what particular model would you recommend for the PHONO amplifier that can be used with LINE-IN input (jack) of my headphone amplifier? I really would like it (Phono-stage-preamp) to be vacuum tube.... Thanks again!

Taking a look at the RM-4+:
RM-4+

It is a standard MM to line-stage phonoamp. However it is missing the low-out MC to MM part. So it should work for your high-out Blue Point MC.