You can adjust the slope on the quarq which will make it accurate in the absolute sense, but it doesn't seem that the issue posed has to do with that.
He wants his powermeters to be consistent, and doesn't seem to care about accuracy. We could start a whole thread on why that's not the question to ask. I agree that knowing that the quarq is ok is a first step, but their slope out of the box from the factory is supposed to be pretty good. Knowing that the quarq is good though won't tell you a thing about whether another crank based meter will be equally good or "closer" to his other devices.
He states that his pt, neo and computrainer are consistent while his quarq is a bit high. If he's in the range of 5% or so, its likely not much more than normal variability and drivetrain loss. If he's training, obviously he can adjust his target power - if he did his test using the pt he simply needs to set his target x% higher on the quarq when he uses that.
The elephant in the room is that its pretty well accepted that while metabolic ftp doesn't change, indoor trainers exert a greater training load at a given target power. Who knows why, really, but most folks that have been using them for a while accept this. So there's a lot of target adjustment going on.
Yes, if you track TSS or use a training load tracker adjusting all this can be a giant pain, which is why coaches prefer that you use one power meter and stick to it. I've been using pm's since square taper wired srms pretty much it, and its only this month that I've had multiple outdoor systems at the same time. Yeah, its a bit of a pain.