Gain and linestage

If I look at the Butler TBD 2250 specs, I see this:
 Input Sensitivity: 2V for 250 Watts into 8 Ohms 
Which I'm sure is the relevant spec to look for on the amp?But for the preamp, what's the relevant spec?  For a Parasound PA6, I see this: Preamplifier Stage Input Sensitivity
300 mV in for 1 V
Total Gain: 10 dB
Max Output: 7.5V Sorry guys.  This one caught me by surprise.  I knew users looked at amp/preamp combos from the "match by sound quality" perspective, but not by this.

Nelson Pass says it well here.

Nelson Pass,
"We’ve got lots of gain in our electronics. More gain than some of us need or want. At least 10 db more.
Think of it this way: If you are running your volume control down around 9 o’clock, you are actually throwing away signal level so that a subsequent gain stage can make it back up.
Routinely DIYers opt to make themselves a “passive preamp” - just an input selector and a volume control.
What could be better? Hardly any noise or distortion added by these simple passive parts. No feedback, no worrying about what type of capacitors – just musical perfection.
And yet there are guys out there who don’t care for the result. “It sucks the life out of the music”, is a commonly heard refrain (really - I’m being serious here!). Maybe they are reacting psychologically to the need to turn the volume control up compared to an active preamp."

And yes he does sell active preamps with lots of gain, to those that want to buy them, remember he’s in business to make money.

Or he's in business because he enjoys doing audio. There *are* other possibilities...

he's in business because he enjoys doing audio.

@atmasphere
  +1111111111111111

And then there's this.
https://forum.audiogon.com/discussions/preamp-output-voltage

There *are* other possibilities...

Of course there are, you’d have to be ignorant to think otherwise.
Some others in the industry are hedging their bets on different circuit configurations, when one is going sour no matter how hard they promote/push it, they will have another to fall back on. It's called business, and doesn't make you any less an audiophile.