I am really struggling with getting a good bass sound in a mix. It sounds fine to me by itself but when the rest of the instruments are in it's a bit of a mess. I've got it sounding ok on a few tracks but it seems I need to use different settings for everything from track to track. My current setup is my p-bass straight into the 1/4" Hi-Z input of an Edirol UA-25EX interface, then into Amplitube/Ampeg SVX plugin. I've been messing with some compression etc to get it to sit more evenly but it seems to either sound pumpy, or too dynamic. I suspect some of it is to do with me being new to bass and not being consistent enough with my playing as well, but strangely I never seemed to have such problems using octaved/pitch shifted normal guitar.

halp recommended EQ/compression settings would probably be good to start me off. Is plugging straight into the 1/4" on my interface the best input method or do you really need some kind of DI box?

I had this huge diatribe written out and lost it being auto-logged out. Anyhow, key points:

Bass is going to have a much wider dynamic swing that a guitar. Even a clean electric. Just the physical nature of the instrument. Even an experienced player with a consistent attack. Pick or fingered doesn't really matter much in terms of actual dynamics if the player is good. (Slap is completely out to lunch dynamically. Massive transient spikes.)

You're fine on the 1/4" input; a plain old DI box isn't necessary unless you're connecting it to a mic preamp. If there's a line/instrument switch for the 1/4" in(s) on your interface, it may or may not be necessary. I find the "INST" switch on my Focusrite- really just a 20dB boost- complete overkill and the line amp totally adequate. I do tend to run bass into other things first, but that's for, y'know, toan.

Re: compression, what are you compressing the bass with at the moment? I tend to prefer a fast attack- sometimes very fast on bass. You may want to allow the initial transient to poke through a bit though; it sounds less obviously squashed. Release time dependent on what's being played;on't set the release so long that the compressor is still clamping down on the next note, though; that's probably not what you're looking for. Ratio... 4:1 is as good a starting point as any; lower or raise as necessary. Threshold, of course, dependent on the signal level feeding the compressor, but if it's up too high that'll produce a similar sort of badness to too long a release time. If you want to squash the bass flat, there's better ways of doing that. Like with a limiter. Don't be afraid to chain a compressor and limiter in series, too. They work well together. If you have a stompbox compressor or a multieffect unit kicking around, you can always compress the bass going in. This is uber-common, and note the profusion of compressors in bass rigs. Speaking of which- some of the models in SVX have compressors and limiters in them. Give these a shot while dialing in the (virtual) amp in addition to whatever "real" compressor you're running. Multiple smaller stages of compression are often better than one big one.

Re: EQ, the bass, IMO, should go where the kick drum isn't. As I prefer the bass to be the deepest element in the track, it generally receives a low end boost somewhere in the 40-50hz region (40hz, incidentally, being the fundamental of the low E on a standard-tuned bass). Kick drum action is happening somewhere above that. 100-300hz is the region where incidents occur between the two of them, and you may find yourself making EQ cuts in places you wouldn't normally think to cut, and then finding the instrument you just cut actually became more prominent in the mix. Blast holes in them and see what pokes through. Mids and top end- and even what's defined as mids and top end on the bass vary according to A) taste and the sort of bass tone you're looking for and B) what's going on in the rest of the mix. Some people reflexively lowpass the shit out of bass guitar. I am not one of them, but if you find it works for you to dump absolutely everything above, say, 5k or whatever, people often do and get away with it.

Hopefully this helps some.