Gain of line preamplifier

Anywhere from 0db(unity gain) typical to McCormack preamps.
Low-gain preamps such as Wyred4Sound will do about 6db,
Medium-gain such as Classe somewhere in 12dB
there are high gain preamps such as AudioResearch, Bryston will blast 20dB of gain
Passive preamp will vary the impedance while active preamp will keep it same. Variable output impedance is heavy disadvantage of any passive preamp even if you have substantially high input impedance of your amplifier.
If there's enough gain from source to poweramp, I'd choose either low-gain or unity-gain active preamp to achieve perfection and Nelson Pass would also agree with me. It will give you larger dynamic headroom and more pleasure at substantially larger volume range to dial REGARDLESSS of quality of passive preamp.
Manufacturers instead of gain would often provide you input sensitivity and max output voltage. 2 ways can be worked around this problem:
1. Contacting manufacturer about gain info
2. Use following formula to calculate gain:
a) Max_Voltage * 0.707 = RMS Voltage
b) 20Log(RMS Voltage/Input Voltage)
Personally, I would prefer purchasing preamps that have DETAILED specifications provided. Ones that aren't provided, would trigger my suspicion that they're not tested properly before going to the market.

Vintage Accuphase E202 integrated have volume control that compensates line vs. phono.
I've used in the past Classe CP30(80's) that also didn't have any problems and volume had been compensated from both outputs.
On the other end, you should look for cartridge with sufficient output so that your line volume level will match to phono.
Separate phono boxes with adjustable gain is certainly best way! I'm not audiophile so I simply loop my phono preamp (Musical Fidelity X-LPS) that somewhat needs a-bit of higher boost vs. line signal through the Nakamichi tape deck that has its own recording level gain stage by simply engaging casset to pause-record mode. This way I get 'step up' stage to compensate with line stage. The electronics of the Nakamichi's recording level are simply superb!

all i stated that unity-gain preamps have advantages of both: transparency of passive and control of active regardless of wires and input/output impedance matching.

George,
I've never quoted Nelson Pass. If 1,2,3 are true, than probably it's a world of dreams. In the real world it's all vary, but far not "most good ones".

I'll add that what can often be an issue with **active** preamps or sources is variation of output impedance as a function of **frequency.** Especially if the design utilizes a coupling capacitor at its output, as most tube preamps do, which commonly results in a large rise in output impedance in the bottom octave.

Almarg, what year/century is it now?? I thought that today we mostly use direct coupled outputs unless it's a tube preamp(many already use direct-coupling as well). It's pefectly legit for me to say that

Most of good preamps are direct coupled and than compare:

1: If using a low (<100ohms) source impedance (which most are).

2: Into a 10kohm passive pot with 1mt of low capacitance interconnect (which most good ones are).

3: Into a power amp with 47kohm or higher input impedance (which most are)

Did you ever check input impedance of Pass poweramps?
Do you think Pass poweramps are good or not?

Are you sure that 10kOhm passive volume control will be able to mute??

Unity gain solid-state preamps such as McCormack, Wyred will kick S to all passives ez and swapping A/B is the best way to hear and know. It's as easy and similar as to swapping Chevy Aveo with BMW Z4 roadster.

Numerically 30...300Ohm output impedance which is substantially less variable vs. passive, but for some reason haven't been presented.