sharke ... A plugin like this would be ideal if, for example, you have a cool sounding bass patch that has some interesting stereo information in the higher frequencies. You could set the threshold on TP Basslane to something like 150Hz, then everything below that frequency (i.e. the bass part) would be perfectly centered while the higher frequency stuff would retain its stereo placement. You can really give your tracks a lot more "oomph" with this plugin, I love it.

Sharke, I like to do that kind of processing inside Rapture, using the Width control.  Any Cake synth featuring Width is fair game, but the multiple filters & extensive DSP routings in Rapture offer a ton of options.
 
There's a quick example around page 7 in the Wide Load - Stereo Wavetables in Rapture 1.1/LE document.  Very versatile.

alewgro
Hello,
 
I have a bunch of tracks with bass and kick drum sounds coming from softsynths.  These synths are stereo.  I want to make my bass sounds to be mono.  What is the best method to achieve this?
 
Thank You,
alewgro

Alright. Here's the simplest way of getting a mono signal from a softsynth:
 
First: Make sure the input of the audio track (connected to the softsynth) only gets a mono-signal from the instrument, by choosing either Left or Right from one of the instrument's outputs:

 
Second: You also need to make the audio track mono. It's possible to have both stereo and mono clips on one single track. It's the output of the channel that decides how they will be played ... Do this by making sure the Interleave button (found on every audio track in the inspector and the console (wide strip)) is set to mono:

(If you don't do this and later freeze the synth, you'll get a stereo file even though you've just selected a mono source as input from the instrument)

Most of the synths I have present a setup dialog that allows the creation of stereo or mono outputs.
 
Since you have to audio, insert the synth again and set created outputs to mono.
Then output the current midi track to the new instance of the synth.
 
Which synths are you using?
 
I think the percussion sounds are mono. The bass sound might actually be stereo depending upon the sample, and/or effects added in the synth patch
 
: a good tool to at least always insert on the master mix bus:
http://www.bozdigitallabs.com/product/panipulator/
 
This plug can flip L/R and it has a MONO switch. Good for all kinds of analysis.

I copy a bass track to another track and use a roll-off cross-over filter (an EQ with the high-end rolled off) on it. Convert the new track to mono. Then use a high-pass filter (another EQ withe the low-end rolled off) on the original track to block the low-end, the same low-end you want on the mono track. Use a gentle slope between the two and see that the filters are generally pretty close to each other. I use a -3 or -6db roll-off for each one. They should overlap evenly, and adjust to ensure all notes are heard.
 
As a test, if you have a bass, or can use a bass-note synth, in a new set of tracks you can play/record all the notes and adjust these options as necessary so that all notes remain the same volume while listening to the mix in stereo.
That can help better tune the cross-over frequency and roll-offs.
 
Beware, though, that when you listen to your mix in Mono for mastering purposes, the low bass notes may be louder after making these changes.
 
You can also try something like this: http://www.nugenaudio.com/monofilter-professional-bass-management-mastering-plugin-aax-au-vst_25#
 
I will use this pretty often on bass, where I want the low end centered (frequencies that are generally non-directional), but pan the mids and highs of the bass to the left or right for placement or effect.
 
 

Thank you everyone for the help!   I don't like to bounce my softsynths to audio until I am ready to do a final mix-down, so what I want to do is have the bass be mono while it is playing from the softsynth.   I will do the mono interlace tip which sounds great!  I appreciate the help from everyone!

I would, and usually do, just use Channel Tools as Craig suggested earlier - pull both the width controls to the centre and you're done

In addition to Channel Tools, X3 Producer bundles the Nomad Factory BT Stereo Imager ST2S-3 which has a Stereo to Mono preset

^^^^
 
I sometimes prefer this one for simpler cases.