Would I be able to use a second device to control software knobs within studio one using the midi learn feature?
1. Yes, as long as you have open MIDI ports you can connect as many devices to Studio One as needed. Just create a "New Surface Controller" and MIDI learn the controls with Control Link. Then assign as needed. Here is a short video on Control Link: http://support.presonus.com/hc/en-us/articles/210041283-Studio-One-Control-Link
Is there a usb mixer/ control surface I can use to control the cutoff control on presence instruments?
2.Yes, Please give Presonus Sales a call: (225).216.7887 ext 3.
While playing my midi instrument (keyboard) is it possible to crossfade between sounds/tracks?
3. If you are playing live and have the track in Studio One playing, you can add MIDI Program Change Automation to switch patches, or Automate the mute/umute of tracks.
If I could crossfade, handle controls like cuttoff, volume etc. and maybe even switch sounds via push of a button though a device external from my keyboard, I'd be very happy. Is this possible? Are there any videos online showing such features?
4. Yes, you can do this. Here are a somesvideos from a quick Google Search, but there are many more:
