As with anything audio, a LOT depends on personal taste and system compatibility, and therefore, comments about which types or models are good or not are just rough generalizations. I "tend" to prefer active tube units over the passives I have heard in my system and that of friends. I also like transformer coupling, which means that the linestage has a transformer at the output and the amp has a transformer at the input.
But, I have heard, and liked transformer-based passives too (Silver Rock). These sound vibrant, lively and deliver the kind of "dense" or saturated sound that I like. I also got to hear the Placette passive linestage (conventional resistor) and the active linestage in my system and preferred the active (I bought the active). My system is not ideal for passives, because of the requirement for a long interconnect between the linestage and amp.
In a friend's system, I heard a linestage built around a light dependent resistor passive. This was an ideal setup for a passive--short interconnects, low output impedance source and high input impedance amp. I liked the sound of the system, but, when we substituted a really nice tube linestage, the dynamics improved markedly. There were three listeners and we all agreed that the tube unit sounded better (including the owner of the custom-built passive unit).
Apart from the sound, some passives present practicality issues that should at least be considered before going that route. Many, particularly of the transformer variety, have too few stepped levels so that it seems like the ideal volume is always somewhere in between steps. To me, the steps should never be greater than 2 db, and 1 db is better. Most do not allow for balance control and most are not remotely controllable (getting the right setting matters a lot to the sound and instantaneous comparison
afforded by remote control is a requirement for that purpose).
There is a form of transformer-based passive that I have only heard briefly that may be of interest. It is one based on an autoformer (kind of transformer). There is a remotely controllable version from Bent Audio that gets around all of the practical limitations of such units--it is remotely controllable, has 1 db steps over a 70 db range and it can control balance. I have not heard it, but, it is based on the autoformer designed by Dave Slagle so it should be pretty good. If you are not technically inclined, you will need someone to custom build a linestage around the module you can purchase. The Bent Audio unit can be found at:
http://www.bentaudio.com/index2.html
