I have a vocal track with the ProChannel engaged, consisting of the Quadcurve EQ followed by the Softube saturation module.
 
If I start with the EQ flatlined, loop a section of the vocal and observing the clip light on the Softube module, there are a couple of times when the light glows orange but that's it - nothing's clipping. 
 
If I engage the HP filter with a slope of 12 or higher, the clip light glows red at a couple of points in the loop. The steeper the slope, the lower the cutoff point at which the clipping happens. At a slope of 12 it starts clipping around 140Hz.  It doesn't matter what mode I have the EQ set to. 
 
So not knowing much about the technical side of pass filters, I'm guessing that there's some kind of boost at the cutoff point, right? Does anyone have any information about how large this boost is and how it relates to the slope? When the slope is at 48, the clipping happens with a cutoff point of only 30Hz, which seems weird because on this track there is little happening below 150Hz, so I'm just wondering what exactly it's boosting. 

No idea of the Q in the HPF, but have you tried eq after distortion rather than before? Saturation usually acts as a compressor as well and tends to flatten out whatever eqing has been done before it.

One way to see what's been boosted would be to drop a spectral analyzer in after the eq, eq out everything that isn't the boosted frequency then ramp up the volume until you can hear what it sounds like.

I usually just experiment with order to see what sounds the best. However I am prone to putting pass filters first in the chain, and inserting other EQ's later for color.

I've been trying to see a boost on the QuadCurve's spectral analyzer but can't see anything obvious.

I find my prochannel modules light up the clip light, if I just sneeze.  I'm not so sure there isn't some tweaking needed in those in an update or whatever.  I can get a fairly frequent clip light, but not hear anything either.... but I'm 61 years old too and have owned Marshalls and Ampegs.

How much headroom below 0dBFS do you have?
 
If you have little or none, it may be as simple as the filter's phase shift causing what was an intersample peak to now be a sample > 0dBFS.

drewfx1
How much headroom below 0dBFS do you have?
 
If you have little or none, it may be as simple as the filter's phase shift causing what was an intersample peak to now be a sample > 0dBFS.

!!!!!!!!!
 
you would like to think the bakers or some other developers would implement isp metering into their DAW's