A tapped single coil is one that has two output wires instead of one - one giving the full output of the pickup, and another giving a bit less.
Let's say a powerful single coil like the Duncan Quarter Pounder has 10,000 turns of wire on it. And let's say that a more traditional Tele pickup has 7500 turns on it. You could create a tapped QP by winding it for 7500 turns, connecting a lead wire to the coil at that point, then winding the remaining 2500 turns and connecting a lead wire to the end of the coil as normal.
You would then use a switching system of your own choosing to choose which of the two output wires was connected, thus controlling the output level from a switch.
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Aside from that, if you want to retain traditional Tele bridge pickup sound, you need a "vintage output" pickup. Putting more turns on the pickup, to bring the volume level up to that of a P-90 in the neck, will reduce treble a bit and increase mids. This is just physics; not a lot you can do about it.
Rather than the QP, which is very loud indeed, have a look at the
Hot For Tele bridge pickup. It's available tapped, and the lower output mode is pretty much a regular Tele bridge. The full output is louder, but not so loud and not so warm as the QP. It should match with a P-90 neck quite nicely, once you balance the heights of the pickups.