No PHONO IN Solution

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.