Amplifier input tech question

In my current system the volume control can only be turned
up to the 9 o'clock position or so, until the music is too
loud. The components are as follows;

cd- Rotel RCD-990 (transport only)
PS Audio DLIII with Cullen stage 4 mods

pre- Parasound JC 2

amp- Aragon 8008st

spks- KEF reference model 3

I am just starting to do my research for upgrading the
amplifier. And, would like to make certain that I select
an amp or amps that are less sensitive than the Aragon.
I have read about the minimum 10 times impedance difference
between pre-out and amp-in. But there must be more to it,
as the JC 2 output imp is stated as < 60 ohms and the
Aragon is listed as 22k ohms. My guess is that there is also a relationship between the preamp output voltage
and the amout of voltage the amp needs for full output.
Is there a formula I can use for finding a good match?