The real problem with the HF-85's touchy gain (and it is surely touchy as designed), is -- once again -- the controversial loudness switch design. When you look at the gain through out the line/tone stage circuits, they are all really pretty normal. The gain of the tone stage itself is <1 at a flat setting, which is true of almost all feedback type tone circuits. In this case, it is more precisely about .90. The line stage has a typical gain of about 14.5, which together produces an overall gain of about 13. This is very similar to that of say a Dynaco Pas preamp, that has an overall line/tone stage gain of 10. The big difference between the two however, is that when the loudness switch in the Pas preamp is turned off, the volume control loudness tap is shunted to ground through it's 4.7K loudness switch resistor, where as in the Eico, that control tap is left open circuit when the loudness switch is off. This has a huge effect on how the control operates through out the first half of it's rotation. This is also why there is such a big drop in gain in the Eico when the loudness switch is engaged.
The easiest solution is to use the same loudness components that Eico used, but connect them as Dyna did in their preamp. That is in essence, that one end of the 10K resistor should be grounded, the other end should go to one end of the .025 cap, whose other end goes to the loudness tap. Then, the loudness switch should act to short out the cap when turned "off", and allow it to be in circuit when turned on. That means that only two of the switch terminals will be used for each channel. If you do this, you will find that all the touchiness of the control is gone, and the loudness switch will operate "normally", without the large change in gain associated with the original design.
Dave