thanks but isnt there more to it than impedance ratios? like gain and sensitivity?@jimbones Yes.
For starters, the output impedance at 20Hz is probably where you should look. With many preamps, the output impedance rises at lower frequencies.
The gain is another matter- much depends on the gain of your amps and the sensitivity of your speakers. As a general rule of thumb, more powerful amps tend to also have more gain as they are used with less sensitive speakers. This should make the gain of the preamp a non-issue, but since the advent of Redbook (which set a standard of 2 volts output for single-ended digital sources) the amount of gain in the preamp has been a bit trickier to sort out. In general though, a reasonable amount of line stage gain is about 15dB. If you are only running digital though, a reasonable amount is about 6-10dB.