Quick bass question............

You shouldn't do.  Guitar rig shouldn't add enough delay to cause any phase problems.
 
The simplest test in most situations would be to reverse the phase of one track and see if it sounds better or worse, and that should answer your question.  (If they are recorded separately as a DI and a miked amp, then you can look at the waveforms to see if the peaks and troughs line up.)
 
I this case I don't think it's necessary.

I always recommend visually (and audibly) lining up the tracks IF there's a phase issue, but I agree with jb101, you really shouldn't have so much of an issue. Having two exact copies of the same signal playing together merely brings up the apparent volume of that signal, nothing more. Having one processed will be just a lesser effect than recording only one track and processing that one track.  If you flip the phase switch on one, you'll hear the bass (at least the low end) nearly disappear, and you likely don't want that. In Mono, the bass would be near-silent, since the two signals simply cancel each other out. 

For tracking, I don't personally recommend going through Guitar Rig during tracking. I recommend processing through Guitar Rig after.

That's actually a great idea CJ, but I'm under a time crunch with this project so it will more than likely be a blended signal of the DI bass with Guitar Rig.

Dbx, you just go out of your sound card and then go into your amp and then go back into your sound card. There is no ground looping issues.

Cj

Cactus Music


CJ, He's taking feedback loop, not grounding. 
Might happen if input echo is on but that all depends on where the track outputs are assigned.   You should use either output 3/4 if you have them to avoid this. 
So if the new track is not assigned to output 3/4 there will be no feedback loop.

There are a few options to beef up the DI bass track, First I'd just try using it alone and create a buss for it and add some EQ and Compression. Unless it was a terrible sounding bass to begin with it should not need much. 

Yep, priceless info there Cactus. I had always wondered why I got a weird sound using 1/2 when doing this. 3/4 or above totally fixed it for me.
 
em: you got all good advice, but here's how I'd do it. Clone the track you have so you have two. Run your plug on the one and mix it/eq it, do the same to the D.I WITHOUT a plug. Just eq and compress etc. If you have a few bucks, definitely go grab IK's SVX bass plug. To me, it's one of the best bass plugs ever made and you can get just about any bass sound known to man. It's that good and should be in your tool box if you can afford it. Good luck!
 
-Danny