I just use fl studio's built-in option and it works great!
It's in the channel settings. "Pro Default" and "Monophonic" mode will retain the quality and you can change it to any bpm.
I'm surprised you're capped out at +-10 bpm.
If you decide to give my method a shot, just watch a youtube video on "time stretching" and it'll lay out what to do. It's quick & simple but much easier to learn by watching it than by reading a few paragraphs from me on how to do it.
Best of luck!