Martin
All amplifiers have some distortion how low that distortion is down to the individual components, design layout construction methods used.
Happily measure any active pre amplifier for you wish totally unbiased, a lot of the transformer type units 'ring' like a bell @ 4Khz and need a serious amount of damping inside the casework, the hand soldered point to point wiring is also left flapping in the breeze we could go into how that generates its own issues. Again implementation is the key
The LDR versions offer a better all round preservative imho if I had to use one it would be this type of passive.
Every-time I have place a passive pre amplifier in my own system, it takes a backwards step in transparency, natural pace (not artificial naim inspired falseness), stage depth and involvement. I have personally had many passive designs over the years including some ultra exotic Japanese uber constructed wonders which cost kidney transplant amounts, beautifully sweet, delicate, nice layering and fluidity in these area's you would struggle to get close even with valves, yet it was like watching a world class painter at work through a first floor window, not all of the perspective was apparent.
We have a couple of customers with the MFA reference unit, it leaves me cold (that doesn't mean its an appealing unit at all for many people), it fits certain genre's of music very well and in their systems does sound good (Not wow my dangles have had an Epiphany or I would trade the wife for it type moment) This goes back to my earlier post, it is what you like not what peter the bell-end's latest bit of equipment is from hifi-dogs bollox forum
However I have a broad spectrum of music I listen to and that type just doesn't cut the mustard with everything I listen to.
I would agree with Marco on one point though
(I know hell freezes over twice this year!), passives upto £2K are generally better than most equivalent active pre's, there are always exceptions, though it is a valid point, (like Hypex Ncore modules/Tannoys/insert any forums favorite toy of the moment I would term them cheap speed a lot of bang for buck.)
Some interesting reading regarding passives
With regard to measurements on the passives I found this on the highly respected Mod squad passive.
Passive pre measurements Stereophile
The channel separation measured on the pre amplifier I use is 0.00025%-0.0005% @ 1khz and 1v even at maximum out (40V I know no one will ever use that level!, oh maybe Marco
) is still 0.093% THD
Before everyone jumps in with because it measure good doesn't mean it sound good totally agreed, however this one does!
Use your ears, in your system that is the only way YOU can decide if it is right for you not Joe blogs forum buddy group.