... Appears that the components affecting the HF are mounted on a little turret board on the front panel near the EQ switch. The HF "pole" components are a 47 NF cap in series with a 1.06K resistor, which gets shunted by a 2.48K resistor when the EQ switch is in the IEC position. Played with adding resistance to the 1.06K and found that an additional 220 ohms upped the HF response so it is now within a dB at 10K and down 2 dB at 20Khz. ...
Actually, this illustrates a big part of why these Repro amp do not have a high degree of adjustability.
For a "perfect" tape head, the equalization in the high frequencies has a single corner at a few kHz; the actual frequency depends on which equalization curve is being followed, This is in the midrange, and tape heads in general have a flat response in the midrange. Consequently, this is not an appropriate place to adjust the high treble - it will unbalance the critical midrange if you make adjustments. The proper reason for such adjustment would be to compensate out-of-tolerance resistors and capacitors, or drifting tube parameters. Modern precision components make it unnecessary, and careful design can make the equalization insensitive to tube parameter changes.
In the case cited here, by changing the resistor you moved the corner so there is a shelving rise in the midrange, which raised the 10kHz and 15kHz levels as well. The measured numerical deviation may be smaller, but the midrange shelving is likely to be more audible.
The very high frequency response is determined by the tape heads - the gap, the inductance, and the parasitic capacitances in particular. In the case of high-output, high-inductance heads which will exhibit an inductance/capacitance resonance in the audible range, the damping of that resonance can be adjusted by the preamp input loading resistance to obtain a modicum of control, and some Repro amps have this adjustment installed. It must be placed at the most critical location, at the preamp input, so a quality fixed resistor is always preferable sonically if it is possible to do so. These heads are most likely to be found on older tape decks with tubed electronics.
Some tape head preamps employ various kinds of equalizer circuits to compensate for poor high-frequency response. Such additional circuitry will inevitably degrade the sound, however subtly. Fortunately, good modern heads of medium sensitivity, such as those used on the Technics 1500, Otari MX5050, Ampex ATR, etc., are now available, and they do not need compensation. If such a head has lost treble response due to wear, it should be re-lapped rather than equalized. Bottlehead sends heads out for re-lapping frequently, and they come back with full response restored. If the response can't be restored, then the head is worn out and should be replaced. Even the best heads cost no more than a Tape Project album or two!
The remaining candidate for adjustment is the bass response where "head bumps" may give a less than flat response. Again, a complex set of parametric equalizers would be needed to fully compensate, and in this case very difficult and time-consuming to adjust correctly. Usually such adjustments are found only in the preamps built into certain high-end tape decks where the designer of hte electronics knows what the head bumps are like because he also specified the tape head to be used, and the electronics are tuned to that particular head (and tape speed). Fortunately (again) extended-response heads with minimal head bumps are available and are a sonically superior solution in our opinion.
Those are the considerations that led to the Bottlehead Repro amp having so few adjustments available.
