I've got a 1995 Subaru Legacy. It's the 2.2 liter, five speed manual. It has developed a stumble between 1k and 2k rpm. It idles fine, runs fine above 2k rpm, but stumbles as it accelerates through the 1k to 2k range. The car has 121k miles on it, and I bought it at 95k. I'm thinking fuel filter is the first suspect. I did change the plugs and wires around 100k.
What do you think? Fuel filter? Something else? Changing the filter looks pretty dang easy so I'm hoping that's it.
Check your mass-air sensor. It might need to be cleaned or replaced. Also, check your O2 sensors. They're not hard to get to, but you will need an O2 sensor socket (for the wiring to stick out). Also, replace your plug wires and plugs! It's so cheap and easy to do, it's stupid - and it is often overlooked as a cause of strangeness at low RPMs (I had the same issue on my Miata for a month before I finally replaced the wires - the problem was immediately remedied!)
If I recall, you might get to one of the sensors from under the hood with a hella-long extension, but I'm not sure. I don't own a Subaru, but a friend does and I help him from time to time.
Mass air certainly could be a possibility. +1 for plugs and wires, and check your knock sensor as well(top of the block by the bell housing, if your EJ22 is the same as my EJ25); if it's original, it'll have failed by now, and they're painfully easy to replace yourself.
+2 for plugs and wires. I replaced mine recenlty on my 2.2L Impreza that helped cure some hesitation at low revs like your car is experiencing. It turns out one of the plugs was not seated tightly enough in the block and some hot air was blowing by it: the whole bottom half of the plug was pitch black with soot. I'll also check my mass-air sensor (I can clean that with carb-cleaner, right?) . I'll check the O2 and knock sensors when I get a new job.
Don't take your plugs out of your head if the engine is hot if you have aluminum heads (and I think you do)! You can strip out the threads and ruin them. Let the engine cool sufficiently before you do this & if you do this.
Don't assume that because you put wires on 20k miles ago they're still ok. Cheap aftermarket wires are notoriously horrible and are geared towards looking like quality replacements much more so than acting like them. Even if plugs and wires aren't your problem, good plugs are cheap and if you bought crappy wires they'll be shot soon anyway. As you mentioned, the fuel filter is another easy one, although this doesn't sound like the culprit from your description above it's probably never been replaced so that's a good one to take care of.
Bryce
Lived with the same problem for quite awhile. Running higher octane fuel sort of helped. Then I replaced the plugs and wires. Wow, problem solved. The wires were the real culprit. Pissed me off I didn't try it long before.
It's going to be electrical, not fuel. The fuel filter would cause it to get worse with higher rpm.
Check the knock sensor(s) I know the early EJ22s had an issue with them. Pulling the plugs and looking at them is always a good idea. Stick with a good brand of wires (NGK) I put cheap ones on my current one and I'm not 100% thrilled with how it's running. but hey it's a $400 car that I've put $400 into and had since november so it really doesn't owe me anything. one of these days I'll pull the code off the ECU. it only comes on after a warm restart :gotme:
By way of follow up, a set of new OEM wires cured it. Going to do the fuel filter anyway now. Thanks for the feedback!
I love it when I'm right. 
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