SonicExplorer
I just tried Voxengo CurvEQ, which I'm already using on the guitar buss.  It has Mid/Side in it.  But can only do either mid or side, not both.

MEqualizer in the free Melda FX bundle supports M/S processing. Each band may be assigned to mid, side or both in the same instance of the plug-in.

IQ4 is very good one (free)
 
http://www.platinumears.com/iq4gui.html
 

I use widening, but I use it as send and on the effects bus where I`m sending it I use channel tools after the widening plugin and channel tools is set to side only (just turn the Mid knob all the way down). That way when in mono the stereo information just collapses completely and doesn't cause any phasing issues. If it`s a mono source then I find the sonitus delay good enough.

SonicExplorer
...Using just side does widen things but gives the impression of shifting the overall tone upward in frequency. I'd have to re-EQ everything with that in mind. 

That's not just your imagination. High frequencies are where we get our width cues, because that's where the L/R differences tend to be naturally. So when you boost the "Side" you're also boosting those high frequencies. In fact, one method for widening is to simply boost high frequencies, never mind M/S. But it's even more dramatic if you can boost highs in the Side using an EQ that lets you apply bands to just Mid or Side. You can then do a complementary cut in the Mid so that the overall tonal shift is less noticeable.
 

bitflipper

SonicExplorer
...Using just side does widen things but gives the impression of shifting the overall tone upward in frequency. I'd have to re-EQ everything with that in mind. 

That's not just your imagination. High frequencies are where we get our width cues, because that's where the L/R differences tend to be naturally. So when you boost the "Side" you're also boosting those high frequencies. In fact, one method for widening is to simply boost high frequencies, never mind M/S. But it's even more dramatic if you can boost highs in the Side using an EQ that lets you apply bands to just Mid or Side. You can then do a complementary cut in the Mid so that the overall tonal shift is less noticeable.
 

Question then:  Is it wrong, or rather inadvisable, to use just "Side" EQ and then tweak accordingly to get back a slightly fuller sound that is lost?  Or would that only serve to counteract the stereo widening effect?   I ask because I have (and often tend to see) a number of VST's, such as EQ's, that offer the ability to switch on either Mid or Side but not both. I'd rather not be chaining together EQ's, or having to make adjustments in two places (mid and side) every time I want to tweak the tone.

SonicExplorer
 I ask because I have (and often tend to see) a number of VST's, such as EQ's, that offer the ability to switch on either Mid or Side but not both.

I have not messed with Voxengo plug-ins for some time but every M/S capable EQ I have from FabFilter, MeldaProduction, Cakewalk and iZotope all allow mid, side and both bands to be mixed in the same instance of their plug-ins. No different than assigning a band left, right or both when the EQ is set to L/R mode.

SonicExplorer
 
...a number of VST's, such as EQ's, that offer the ability to switch on either Mid or Side but not both. 

M+S would be normal stereo filters.

BTW, if you want something like Channel Tools for XP try StereoChannel from Sleepy-Time DSP. Development stopped some time ago and their site is no more but the plug-ins are still around. I found the collection at the Bedroom Producers Blog. GVST plug-ins may be of interest. There are some stereo processors on their beta page. Although listed as beta, they have been around for years.

bitflipper

SonicExplorer
 
...a number of VST's, such as EQ's, that offer the ability to switch on either Mid or Side but not both. 

M+S would be normal stereo filters.

Clarification: When I said "I have a number of VST's, such as EQ's, that offer the ability to switch on either Mid or Side but not both" what I really meant was the ability to EQ individually.  For example Voxengo CurvEQ can do only mid or side, or both together (normal EQ mode) but does not offer the ability to separately EQ both the mid AND the side.  This is why my follow-up question came in "Is it wrong, or rather inadvisable, to use just "Side" EQ and then tweak accordingly to get back a slightly fuller sound that is lost?  Or would that only serve to counteract the stereo widening effect?"

This is a usefull introduction to mid/side recording and processing

https://www.uaudio.com/bl./mid-side-mic-recording/

Plugins which only work on the mid or the side but not both are generally intended to be used after splitting the mid and side signals into two tracks, or for working with audio recorded to two tracks using a mid/side mic setup.

I was once told by a BBC engineer that if asked to work on mid/side recordings the first step would usually be to collapse the tracks to mono...