With the OTR10 and 12 machines, there is plenty of room for isolated RCA jacks to be installed only inches away from the playback head. You can use a ZP3 with the Tape Option and plug the heads directly into it with a good shielded RCA terminated microphone cable.
The cards on the front of the machine for the left and right channel can simply be pulled out a few inches to disconnect them when you are using the tube playback electronics.
This setup, even with the stock heads gets everything really right with impeccable staging and imaging that comes with single ended triode circuits that employ no negative feedback.
The ZP3 is infinitely adjustable from RIAA to a reference NAB and beyond to IEC. As you adjust more towards the IEC calibration the ZP3 can add a mild bass boost depending on the input impedance setting for the heads ( a switch on the ZP3). This can be and is a handy tool for playback of European tapes with the IEC equalization since you effect the bass drive as well as add or subtract as much treble as you want.
From there the focus is on tubes for the ZP3, and if you want to go further, you can install aftermarket heads and at the same time have the headblock laser aligned. Getting the heads aligned perfectly is far more important than the pedigree of the heads themselves. Having both is the ideal.
When you consider the overwhelming adjustments on the OTR models a nice contrast is having a simple tube playback amplifier with nothing more than 1 switch for bass drive, and a treble control for each channel (if you want to look it that way) which gives you everything you need to instantly make any tape sound good without a bunch of confusion.
When comparing it with more expensive active equalization coming from feedback, the ZP3 creates a more believable experience by virtue of it's balance in both frequency and dynamics, but equally important in the arc of sound they reproduce. It is like a CNC machine plotting 3 dimensional space on an X Y Z grid. So precise, and so believable. Nothing is larger or smaller than it should be, nothing is more forward or more reserved than it was... it's just incredibly right. In contrast, the feedback designs sound artificially expanded where they shout at you unbalancing the harmony of the layers. On the surface for the first few minutes you would interpret it as better ( and in every bar you would interpret it as better) but for the connoisseur of perfect audio it's a pretty typical let down in so much as it gets so close to actually right, but fails to achieve.
Hows that