No PHONO IN Solution

The headphone amplifier I use doesn't have PHONO-IN input.
I always thought that MC/MM phono preamp such as Music Reference RM-4 would be sufficient. But as it turned to be the device like this only brings the signal up to the level when it can be fed into the PHONO IN input of the amplifier that does already has a PHONO-IN jack. I wonder what I supposed to do if the amplifier doesn't have PHONO IN build in. What category of devices can be used?
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+:

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.
If your cart output is really 0.25 mv it WILL need either a step up of some kind (transformer or pre-preamp or active gain stage) or a full function pre-amp that can handle a low output MC. Anything much lower than 0.2 mv would be a very low output MC. Typical output voltage for a high output MC or an MM/MI is (I think) around 5 mv. However if your headphone amp is line stage only, you will also need a phono stage to provide RIAA equalization and gain. There are a number of great tube phono stages that can handle a low output MC cart w/o a seperate step up. What's your budget?
I feel comfortable spending no more than 1000 dollars on phono stage. What would you recommend?
The Sumico Blue Point 2 has an output of 2.5 mV which qualifies it for a MM stage.
As far as i can tell, you should have no problem running the RM4 (looking at the RM4+ specs above) into any input on your linestage but I note the RM4 does not have a pre-pre stage to accept low-output MC cartridges, which, as Swampwalker suggests, means you need a step-up or headamp or a different one-box phono stage which includes one (still assuming you want to run a low output MC cart). There are any number of step-up devices available, new and used. I personally use a headamp these days but there are people in both camps. There are some inexpensive headamps and SUTs out there to add to the RM4 if you want. Many like Cinemag SUTs. Some people have used the Hagerman Piccolo headamp to good effect. Some have hotrodded it a bit too.