Yes!! Never run the head with only one side plugged in, you'll eventually fry an output transformer or something in the amp section. Download the manual from line6.com.
On the volume sensitivity - remember there are 2 volume controls - channel volume and main volume. If your channel volume is cranked all the way (and the gain, too), then, YES, the main volume control will be super-sensitive!
If you plan on using this live, get yourself a FBV MkII Shortboard controller - it lets you scroll through the banks of user presets as well as select the 4 channels in each band AND also gives you individual on/off control of various functions like boost, distortion, etc. And a volume/wah pedal.
The FBV MkII Express will let you change between channels on theamp-selected user bank, and has a wah/vol pedal and is only $99. Both FBV MkIIs will let you plug the amp into a computer for updating software, downloading more preset sounds (and FX), and you are able to save your all created (or just saved) user presets to the computer for 'safekeeping'.
Note there are FBV Mk1 controllers out there that are not compatible with
the Spider IV, so beware of buying an old one used.