Marion- A pre-amp is needed to boost the signal from passive pickups to the amplifier. Without a pre-amp, the volume isn't adequate for performing. This is true for under the saddle, and the various contact type pickups, regardless of the brand. This is why some companies have put holes in the sides of their guitars, putting in a panel to control the preamp, often with EQ to vary the output of the pickup.

Other companies, including PUTW, make external pre-amps, which offer the flexibility of being able to be used on more than one instrument as opposed to the ones internally installed in guitars. Features and cost vary widely from small boxes which will fit in your case, to rack mount units costing big bucks.

But in any case, pickups like PUTW, I-Beam, and all the various under the saddle pickups require a preamp. No getting around it unless your amplifier has a pre-amp stage built in. For your situation, an external pre-amp is the way to go. One pre-amp for multiple instruments. The PUTW Line Driver is small, offers a ton of clean gain, with an on/off/volume switch, and will fit in your guitar case, possibly even your violin case. The Baggs Para Acoustic DI, (PADI), is very popular, offers more controls and is a very fine unit, with about the same gain as the PUTW pre-amps. It is larger, and costs a bit more, and may or may not fit into your guitar case. Probably not. But if you don't mind carrying it, the PADI is a very good pre-amp, and is readily available in many music stores.

Hope this helps. Compare costs and features, and what fits with what you want. Feel free to PM me with any questions. Thanks, BlueJay