George is correct in that if a preamp imposes a coloration a passive control, even a very inexpensive one might be better.
It's not cast in concrete though- passives have problems of their own that have been expounded upon a lot over the years, just like active preamps.
If OTHO if an active preamp is free of many of the pitfalls that a line section has to face, it will be found to be more transparent.
The line section/passive control has 4 functions:
1) Control volume
2) select input
3) provide any needed gain (with a passive of course if set up right you won't been needing any)
4) control the interconnect cable from the preamp to amp.
It is 4) on this list that is the least understood- heck, most audiophiles don't even think that is possible and wind up buying some really expensive cables to deal with/prevent cable artifact. This is also the thing that passive systems don't do at all (essentially its mathematically impossible, and many line sections don't do very well with this either). But if the line section **does** prevent cable interactions from occurring, it is very likely to also be the most transparent; introducing the least coloration.
