A good preamp can introduce tubes into the sound. I also have a passive pre with the same Alps Blue. The tubes add a subtle difference that can be varied by rolling tubes. Yet, the passive is no slouch either. I am running balanced interconnects. Your mileage may vary.
Why do people buy preamps if they can directly connect amp to stereo source/phone/PC/tv?
The purpose of preamp as I know it is to boost the signal to make it stronger. But I have never used preamps and my current system is already too loud (and sounds great) at moderate volume without preamp. Why do I see many people buying preamps even though they have powerful power amps?