I personally prefer more upbeat music to listen to when studying. but like you said, combining different types changes the pace a bit. I noticed the rest have given quite a lot of good ones, so I'm sharing 2 that I find quite nice. First is from an anime Famillar Myth -Heroic Desire https://www.youtube.com/watch?v=pWlfyXi0KYs
Second is from a vita game - Freedom Wars. Personally my favourite but that is very subjective since I heard this music under the ideal conditions for the first time. BTR - https://www.youtube.com/watch?v=uPSXaqW0CyA
Might not be of much help since these are from my tastes.
Edit: if you really wanna hear music to study, I really do suggest getting a decent set of earphone/ headphones and listening to 320kbps music. Higher quality music is more relaxing to the ears I feel. Plus if i'm not wrong youtube limits the bitrate roughly to 126 kbps. Once you properly hear 320kbps on a decent set of earphones/headphones. It's a quite a difference. Makes listening for long hours less stressful for the ears. Spoken from personal experience.