High Gain?

You know it will probably work with most.  This advice is probably to get you to avoid using passive preamps.

Almost all active preamps, in my mind, have too much gain, so they'll drive a FW just fine. Just avoid anything fussy or sold as low-gain.

garyalex High Gain?

I’ve read in more than one thread that the First Watt SIT-3 should be used with a high-gain preamp. I’m wondering what the number is that determines whether a preamp’s line section is considered to be high gain or not?

It will all depend on how much your source gives out, and how efficient your speakers are. You can clip the FW Sit-3 with just 3v in from the source

Try this put your source direct into your Sit-3 and play a known track that starts of very quite and builds up.
Then you’ll know how much "if any" gain a pre will need, you could get away with just a $49 Schiit Sys passive or low gain Schiit Freya pre

Cheers George

There is usually a spec titled Gain.  It is usually given in both preamp and amp specs.  It can vary from 3dB to 29dB and probably higher in preamps.

Most preamps are higher gain and, actually, having too much gain in our preamps and amps is more of a problem than having too little.  When you have too much gain you keep your volume control below, say, 9 o'clock.  This can result in detail being lost.

If Pass says to use a high gain preamp,  I'd look for one with a Gain spec of 20dB or more, and there are a lot of them around.