I'm running a similar setup, and don't have it all ironed out yet but I can tell you what has made improvements for me.
I'm running a Meade 2" 2x teleextender and running at 80mm-f/20 with one scope, and 100mm-f/18 with the other. I have the 2" eyepiece holder for the Quark (this: http://agenaastro.com/daystar-2-inch-ey ... sz1d2.html
I have the snout of the PST etalon in the eyepiece holder on the Quark, so it's behind the Quark in the optical train as Mark mentioned. At this point, I'm literally just holding my eyepieces by hand against the PST etalon, but I have a few ideas on how I can actually mount them.
The view on inital testing was extremely dim. I could barely see the Sun at all. A few very tiny changes have made it much, much better.
First, I went through and made sure everything was square (the diagonal to the focuser, the telecentric to the diagonal, the Quark in the telecentric, etc). It was all very very small tweaks, but it made an immediate improvement.
Next was to rotate the PST relative to the Quark. The etalons polarize the light strongly, and I think there was a bit of light reduction here as a result. This was less of an improvement than getting everything square, but it did make an improvement (it may have been due to just continuing to make everything more square, though...I'm just not sure).
Finally, going back and playing with tuning the Quark and tuning the PST made the biggest improvement. I went back to -5 on the Quark, then tried adjust the PST tuning. Then -4, and adjust the PST. On and on, through every setting, until I found what worked best. Each and every Quark and each and every PST is different, so you'll need to test yours out. This made a huge, huge, huge difference--the Sun is easy to see now, as opposed to nearly invisible. I strongly recommend you pull the tuning ring off of the PST so you can tune it further in either direction than you can when the ring is mounted.
Hopefully these things help out! Let us know how it develops.