passive
Banpe
I've been using passives since the late 80's and am hooked. If you don't need all the traditional features of a pre-amp eg tone controls, tape loops etc and your sources have a sufficient output, they are definitely worth investigating. They are typically very accurate, but can be a little lifeless depending on the model chosen.
Because they are pretty simple in concept, many people construct their own, but you can also find kits, "assembled kits", fully-finished units sold direct and fully-finished units sold via distributors and retailers. Each route has its advantages and disadvantages.
As has already been mentioned, there are two basic types in general use A) resistive and B) transformer based (TVC)
A) Resistive: Again these come in several flavours
A.1) Based on a potentiometer: These give an (almost) continuous variation in attenuation. Very easy to construct for yourself, but there are some very well-respected, fully-finished units. QED used to make a version which was very cheap.
A.2) Stepped attenuators: These use a switch and a series of resistors to reduce the signal. Again there are two types!
A.2.a) A string of resistors are wired in series with taps in between. The switch just selects one of these taps. A potential disadvantage is the number of solder joints the signal has to pass through
A.2.b) Stepped ladder attenuators: In the switch is arranged such that there are only ever two resistors in the path of the input signal and one in path of the ouput signal. This is theoretically better, but whether it is so in practice is another matter. This type uses more resitors is so potentially more expensive if you are using fancy resistors.
A key decision here is the value you choose; 10kohm or 20kohm are typical for solid-state amps. The higher the value, the easier it is for your source to drive, but the more risk of rolling-off high frequencies- particularly if you use high capacitance interconnects from your pre to the power amp.
B) TVC:
The main advantage claimed for TVCs is that they do a better job with impedence matching, so less chance of high-frequency loss.
Again two types.
B.1) Based on autoformers. There is one coil and the output is taken from various taps along its length.
B.2) Based on tranformers. These potentially offer: complete iisolation ftom the incoming signal, eathing flexibility, conversion from balanced to unbalanced and vice versa, and even gain ie the signal can be amplified passively.
I'd be inclined to start with some thing simple and cheap, if you have to buy, or whatever you can borrow to get a bit of a feel for passives and then decide which of the various flavours you want to try.
FWIW (for what its worth) I started with a very cheap QED based on a potentiometer (still kicking around somewhere), then moved on to a stepped ladder attentuator with fancy components and an awful Hammond enclosure (ditto). I've now moved on to TVCs which I find give a more lively/energetic sound (yet retain the detail) even though- in theory- I should not have had any roll-off problems with the stepped attenuator. I currently own a Promitheus, but am evaluating another make, because, unfortunately, they don't all sound the same!
As was said earlier, if your Wadia volume control works by reducing the digital signal, then using an external passive should sound better. (but you'll never know till you try it!)
Good luck