Bob Gollihur's site has a good explanation of why impedance matters and why you may need an impedance matching preamp for your piezo pickup
https://gollihurmusic.com/ohms-impedance-from-the-input-perspective/?srsltid=AfmBOoqHLBKvd9-BHmshssKiCIrAT5-J3dtKmh8iyoYJAZfJ0RjsUZO7
If your Shadow 950 sounds nasal and thin through your Rumble 115, then you probably need a preamp with a high (5-10 MOhm) input impedance to match the high impedance of the pickup.
The simplest preamps, like the K&K pure preamp will buffer the impedance difference between the pickup and the amp input but offer no EQ, phase inversion or high pass filter (HPF). If you can afford it, I would recommend a preamp with a HPF control as that will help defeat feedback on loud or boomy stages.
Again, Bob Gollihur's site has a good range of preamps available, but it's a USA site, so it's worth seeing waht's available from a european dealer
https://gollihurmusic.com/preamplifiers-and-eq/1-channel-preamplifiers/