The big issue you will find is as the load of the amp ( 8 or 16 ohms ) changes so will the power output and the "clean" headroom. Most of the tube amps we know and love with an impedance select switch change the bias level of the final power stage to avoid going into saturation ( distortion ) too soon and over heating the tubes.
In the event you are a clean tones player ( not the Arcon's selling point ) the higher impedance is a better choice as this gives the amp a lot more headroom before saturating. If your Archon has a switch to allow an 8 ohm load it will break up sooner and at lower volume levels with the 8 ohm speaker typically...some amps compensate to try and perform the same regardless of load but not most tube amps.
Now if it does NOT have a switch and you run an 8 ohm speaker when the final stage is needing 16 ohm you will cause 2X the power to travel into the output stage. This will cause a lot of heat and a lot of distortion very early in the turning of the volume knob. It will sound awesome....right before all the magic smoke pours out of the cabinet!
Always remember... Power = voltage squared / impedance...so as impedance halves power output doubles.
Hope that wasn't too nerdy.
Steve