It must be a frequencies issues as much as just a pure output issue.
While that might factor into it a tiny bit, I assure you 95% of your problem is output - a "real" singlecoil will get overpowered by most humbuckers, as well.
I've, currently, got an Air Norton 7 and a Tone Zone 7 in my Suhr, and there's a definite volume drop when I pull the coil tap vs when I have the full humbuckers running. These are pickups that are "designed to be coil tapped," inasmuch as any 4-conductor humbucker is or is not "designed" to be coil tapped - it's simple physics, a single coil has fewer magnets and a smaller amount of copper around it than two coils wired in series.
For me, what matters isn't that the humbuckers and tapped humbuckers aren't "equivalent" in volume, so much as a medium-hot set of humbuckers, when split, gives me about the output of a medium-hot singlecoil, so the playing response feels about right.
Another thing you could try is wiring the humbuckers either in parallel, or with a series/parallel switch - parallel wiring cuts the output and gives you more of the "sound" of a singlecoil, but still bucks hum. I prefer a full split, but some people prefer this as a better, noiseless compromise, and IIRC it should be a little hotter than a split coil, though still not as hot as a series humbucker.
I know this isn't what you're asking, but I just want to explain that you're basically chasing a ghost here, trying to get singlecoil and humbucker tones out of the same pickup with no change in output. You'd need some sort of an active preamp to do that, and maybe that's worth exploring, a coil tap that also pulls a preamp into the circuit, with some sort of trim pot that you can adjust to get balanced output between the two.