I got a reply
Dear Sir,
Thanks for your email.
We fully understood the input enable/disable function that you asked for.
However, due to the limitation of the hardware, it is extremely difficult to implement the function on HP-A8.
Instead, we have added a new menu called "INPUT SEL MODE" since V3.00 firmware onwards.
There are two choices in this setting.
One is the INCREMENT and the other DIRECT.
INCREMENT is the default setting and is the same as before.
Every time you rotate the INPUT SEL knob, HP-A8 changes various settings internally so that a signal fed to a selected input terminal can be approprioately received.
As you noticed, the drawback is that it takes time to access to a desired input.
DIRECT is a new way of selecting the input.
Since HP-A8 does not change the internal settings until the INPUT SEL knob is pressed, you can access to a desired input swiftly.
After you select the desired by rotating the INPUT SEL knob and then press the INPUT SEL knob, HP-A8 changes the internal setting.
Could you please try to use the "DIRECT" INPUT SEL MODE?
Thank you and best regards,