I tested with Perfect Space and the new L-Phase Multiband. At Medium or High Precision settings, the L-Phase Multiband induces so much PDC that it will manifest as a significant timing sync error - not just a phase error - if PDC isn't working. But it worked fine.
 
The fact that you're only getting a phase error makes me wonder if the Waves plugins are misreporting PDC by a few samples or doing something else to the signal that manifests as a phase error when used in parallel.
 
The only time I ever heard a phase error due to PDC was with Addictive Drums sending to parallel compression because SONAR wasn't properly compensating the 64 samples of PDC that AD's own internal FX induce. This has since been fixed.

yes, if it's plugin specific could it be to do with what/how the delay is reported by the plugin to the host?

I tested with Sonitus Multiband. I think any oddity you hear with that plugin is due to the way it alters the sound even when it's nominally set to have no effect. I tested in another project I have that measures PDC, and Sonitus Multiband doesn't induce PDC unless the Limiter is engaged; then it introduces 64 samples of delay. But when using it as a parallel compressor, it causes the same muffling/phasing of the signal, whether the Limiter is engaged or not. My guess is that this is because it's not a Linear Phase processor.
 
If you A/B the Sonitus Compressor output with the dry signal, it sounds pretty much 'unmolested', but the two cannot be made to null even if you add a sample delay (e.g. Channel Tools) to try to compensate for faulty PDC. No matter what you do, there is always some incomplete nulling at some frequencies.
 
It makes sense that parallel compression requires using a linear phase compressor to avoid this. I don't know if those Waves plugs are linear phase...?
 
EDIT:  I confirmed this hypothesis by setting the L-Phase MB precision to non-linear. It has an effect similar to Sonitus MB even when it's not doing anything - i.e. bands are added but with no parameters altered. Invert the Aux track output and set the LP MB precision to Low/Med/High, and it nulls perfectly with the dry signal.

Great contribution and effort, Brundle.  Thank you.  I, too, tried LP MB and it works fine.  I'm not sure the C6 and C4 are linear-phase items.  If THAT is the situation, then I've certainly learned something.  Other nice Waves compressors work, like the whole LL Multi and Maxi series.  It might indeed be a phase thing that PDC doesn't handle. Pretty deep, and the consolation is that I'll not be too terribly restricted by not using an exotic multiband like C6 in parallel compression.  
 
Best of regards to you and all who responded here!
 
Jon

Glad to help. It was a learning experience for me, too. Makes me wonder how many 'golden ears' are out there right now rockin' to the phase error of parallel compression using non-linear plugins. 

Oh, Lord, it's not that subtle, I don't think!  And the way the system got confused after deactivating the plugin was a mystery, too, until I realized that that audio engine needs to reset/restart to get "its head on straight"!   I tried this as well in the ProChannel strip and couldn't get any available plugins to mess it up.
 
What is your standard parallel compression setup, if I may ask.  I'll send virtual cash out to Colorado!

I've never really used parallel compression 'in anger'; only played around with it. My mixing efforts seldom reach that level of complexity. I try not to beat myself up about it. 
 
You can start by Googling 'parallel compression site:forum.cakewalk.com' to find discussions on the forum. Some of the hits i got from this reference external articles/tutorials on the subject.

Using multi-band compressors in parallel is asking for trouble. I wouldn't recommend it, for the very reasons you cite. Each of the nine crossover filters will introduce phase shifts, even with all controls zeroed. AFAIK there is no way to compensate for crossover distortion via PDC.
 
Parallel compression on drum busses is a common effect, but it'll always be broadband compression. If you're using a multi-band in order to avoid issues with the compressor being overly-sensitive to low frequencies, use a compressor with a sidechain filter instead, since filtering the sidechain will not result in phase shifts.
 

bitflipper
Using multi-band compressors in parallel is asking for trouble. I wouldn't recommend it, for the very reasons you cite. Each of the nine crossover filters will introduce phase shifts, even with all controls zeroed. AFAIK there is no way to compensate for crossover distortion via PDC.
 
Parallel compression on drum busses is a common effect, but it'll always be broadband compression. If you're using a multi-band in order to avoid issues with the compressor being overly-sensitive to low frequencies, use a compressor with a sidechain filter instead, since filtering the sidechain will not result in phase shifts.
 

Bit, what are you referring to as parallel compression in this example?  Do you mean two multi-bands in tandem?  I was not doing that.  My aux bus just acts like it can't handle some of the multi-band plugins, although the new LP MB and many other compressors work.  I'm curious what the core functional culprit really is.  If it is not the center timing of the signal, maybe indeed it is complex phase issues (timing, essentially) across the multibands which a-phase with the original signal in the same output bus.  Puzzling.  Maybe you meant putting a multi-band-processed signal in parallel with its source.  That would make sense.
 
Thanks for prior and any future dialog.  I'm the type that likes to get down to the phenomenon in play on this kind of stuff, but I don't have the detailed savvy to call this one!

For parallel compression you would only want to use an ordinary old compressor. A multi-band is for other things. For what it's worth, I use a ton of Waves plugins and have not seen the issue you describe.
 
Dan