TIP: When you ask specific questions, give specific information. Telling everyone what equipment you are using will help the process of communication.

That being said, you seem to have answered your own question. If you have found the best sound, that is because you went through the process of elimination. If you have three volume controls (mic channel, mixer main, computer input) you simply want unity gain on the two with the most noise; and higher-than-unity gain on the remaining one. If this is what you did...right on. Also, NEVER run the line-level outs of a mixer or anything else into the "mic" input of your computer. You probably know this, but this happens all the time and I am just compensating for a lack of description.

As far as the bass goes, do not overcompensate for poor monitoring. If you have a kick-ass pair of A.D.A.M mastering house monitors...I apologize. But I am assuming you don't (who does?), so when you are mixing your songs shoot for a balanced frequency response more than "punched in the face" basslines. If your mix is balanced properly it will knock people out anyway when they put it through their systems with the 15's in the back of a Civic or whatever. For assistance in checking the frequency response of a mix, strap Voxengo SPAN across your master channel effects:

http://voxengo.com/freevst/#VoxengoSPANVST

This might not work for you, because I have no way to know if you are using VST-compatible software. You can also try adding higher-frequency notes that duplicate the basslines or running the bass through tube-amp effects to get harmonic distortion. One way or another, you can also listen to other artists through your monitors and make yours comparable. If it starts to sound like "Baduism" by Erykah Badu...or "Asian Massive" (by DJ ?), you have gone too far. Anyway, hope something here helps.

Give the vocals their space. It seems mixing is the most overlooked concept of mixing.

Use direct out or the insert point. You dont need to go trough the hole mixer when recording 1 track. this will eliminate alot of noise.

One key trick to mixing, is to manipulate the various instruments tonal characteristics so that they don't "step" on anything else.
If you have a problem with adjusting the bass volume try a different bass tone. Or, if you don't want to re-take the track, try using a parametric eq. Bring the bass up in the mix to where you want it, and then sweep the eq until the vocals start to stand out better. Listen to the bass again to make sure you can hear all the notes, and then adjust volume to taste. I think it sounds like you may have too much lower midrange or upper bass in the bass tone and it's interfering with the lower component of the vocal sound. As an aside, the lower component of the vocal sound is where we percieve it's power to be. Try it.