Playback through computer speakers

Not if you are using ASIO. That allows only one active interface at the same time. You could use another method (I'm not sure what it is called as I'm not at my DAW) that does allow it to do this.
 
However, usually when you just increase volume when panning, it means your headphone connection is either receiving a mono signal (which then just increases or decreases the total volume as it adds both channels), or the cable for your headphones isn't plugged in fully, which basically also results in a mono signal (as it touches both 'pins' in the connection at the same time).
 
I would check if your interface indeed has a stereo headphone output (which I would think it has, but it doesn't hurt to check) and if it does, maybe a setting in your audio interfaces control settings is not set correctly. And lastly, if you are using a bus to send your mix through to the headphone output (if possible) check that it isn't a mono signal.

a couple adapters and you could hook them up through the "Main outs" on the back of your IONE.
(A 2 prong stereo Male 1/4 apt to an 1/8 female)
 or run a stereo 1/4 male from your headphone jack to a 1/8 female for your PC Speakers.

But as Chuck said easy enough to unplug your headphones and plug the computer speakers instead. Unless the connection is hard to get at all the time. In that case a little extension cable running the back of your CPU.