I've been reading some of these discussions,
and had to chime in here, as I've been trying to get to the bottom of what
constitutes a good match preamp to amp as well.
I am finding it comes down to gain, output impedence of preamp to
input impedence of amp, voltage output of preamp, and input
sensitivity of amplifier. Transconductance apparently comes into play with tube
preamps as well.
Here is a link from Steve Deckert at Decware that describes these factors: https://www.decware.com/paper55.htm
I currently have a Decware Torii Junior, 20 watt tube amp, and Canary Audio C630 tube preamp. Looking to replace with a Rogue Audio RP-7.
Cannot get above 86dB spl at 10’ without pushing both units above 70% full throttle on some recordings. But at these sound levels, the music is liquid velvet, gorgeous. Just want a little more headroom, and volume.
Canary output impedence: 1600 ohms; Torii Jr Input impedence 100k ohms
Also, Decware amps are set to 2 volt input sensitivity, compared to industry standard of 1 volt or less.
These two specs reveal this is probably not a good match. Lower output impedence (50-100 ohms) matched with a high input impedence (100k+) is a happier balance. Higher max output voltage on preamp and higher gain helps as well.
Of course for those of you who have 100 watt+ amplifiers, this is all a moot point.
Enjoy hearing good sound! Like a gourmet meal!