Check this table for the right frequencies to emphasize. I stole it from a post in the propellerhead forum:

try a HPF + new strings

Whoa... you don't have access to the Sonar plugs? Those are a huge part of the value of Sonar. You're missing out, man. You should call Cakewalk tech support about that. You'll likely get much better results with the goodies.

Nice one, Zonno. I'm not only bookmarking this thread now I'm making notepad doc out of that. Thanks.

Compression really helps on the bass.
 
I like using the ProChannel concrete Limiter with the bass switch turned on.
 
Using the Sat knob to add just a little saturation helps it cut through.
 
EQ is important.  It does depend on the sound, but a few starting points I use are:
 
  Remove everything below 41.2HZ (low E) if in standard tuning.  Probably even higher.
  Low Pass Filter 3 - 5KHz
  Attenuate around 300Hz to improve clarity.
 
Experiment, but these are just rough starting points that work for me.  It depends on the recording, the style of music (e.g. slap bass will be very different to Jazz), etc.
 
 
YMMV
 

3) Using an Avalon U5 preamp / Direct 4) P-Bass with SD Quater Pounders pickups

Those two things should get you most of the way there.
The U5 is a fantastic DI.

If you're wanting an "amped up" kinda sound, checkout the Ampeg Model in GR5 or Ampeg SVX.

I'd high-pass the guitars (not much needed below 100Hz in a band context... and run a slight high-pass on the bass (usually somewhere between 25-35Hz). This helps create separation from the kick.

If you're listening to straight DI of a passive P-bass, it'll need compressed and a little EQ to sit more like a mic'd amp.

The positive is that you have good fundamental tools.  

Here's one strategy for producing bass guitar. Put your DI bass git on one track and your amped bass on another. If you don't have an amped bass copy your DI and run it through an amp sim or an IR. Copy the DI to a third track and name it "fuzz" or "bite" so you end up with three tracks, DI, Amped and fuzz.

On the DI track drop a fast FET compressor (1176, PC76 etc) on it and compress a fair amount. I sometimes like to add some wooliness by turning the attack and release to their fastest settings. This track will provide the presence and solidity of your bass guitar line.

On the Amp track compress a bit but don't go too fast on attack and release. This should be your big, round, low bass sound. I think of this track as being most like what I hear with a bass guitar playing through an amp in a room.

On the fuzz track drop on a tube simulater (the TL64 does a pretty good job with this) and drive it so you're really fuzzy. EQ out the low end so that you're really only getting mid/mid-hi's and above. This track provides bite and definition.

Route all three tracks to a bus and mix each of them to sculpt your sound. I usually like some kind of LA2A-type compressor (TLA-100, PC2A etc) as well. Next, put an EQ on the bus and use a hi-pass filter at about 500hz with a really gentle slope...on the ProChannel EQ I usually use a 6. Drop the gain on that filter all the way down so you're only left with the frequencies above it. Sculpt the mids and hi's and when done slowly dial back in the low end. I usually dial back in the lows with the rest of the material playing to get the balance right. Potentially carve out a space for your kick with the other bands of eq. Adjust futher where where needed. Use the tracks feeding the bus to change the character of the bass sound. Don't be afraid to compress a fair amount and don't be afraid to automate the relative levels of the different bass tracks..sometimes the character of the sound needs to change (from verse to chorus for example).

Good luck,
Dean

After you follow everyone else's fine advice, try using this

On the fuzz track drop on a tube simulater (the TL64 does a pretty good job with this) and drive it so you're really fuzzy. EQ out the low end so that you're really only getting mid/mid-hi's and above. This track provides bite and definition.

That's a cool tip!
This can net you that old-school Chris Squire, Geddy Lee type bass tone.
Distorted mids/highs... which adds character... without a muddy/fuzzy bottom.

My basic, "generic" tips are: Lots of compression to counteract response problems in the playback medium, finding a mid frequency and boosting it to bring up pick noise, and EQing bass and kick as a pair so they carve out their own parts of the spectrum. Also, change strings more than once a decade FWIW I've used V-Vocal with mono synth bass parts not so much for tuning, but to add slides and other little coolnesses, like slight upwards pitch sweeps to the note.