In general, you do not want to preheat aluminum alloys. They are strengthened by heat treatment or by cold working. Both means of strengthening will be lost if the material is held at temperature for any length of time.
The tensile strength of 6061-T6 is on the order of 42 ksi in the as wrought condition. If you weld it successfully and in doing so, minimize the time at temperature, the weld is considered good if you can get it to pull at 24 ksi. In other words, even if you do everything right, the codes expect 6061 to lose about a 40% of its tensile strength.
I had the best results when I used heat sinks and blowers (fans)to minimize the time at temperature while welding and I never used preheat. If you are experiencing fusion type discontinuities, you need more heat input, i.e., Heat input = volts time amps divided by travel speed. Notice there is no place to factor in preheat temperature - don't!
The basics mentioned are applicable for both GTAW and GMAW.
Best regards - Al
Oops. Yes, I meant interpass temp., not preheat.
As far as preheating Al goes, I wouldn't do more than enough to dehumidify the surface.
Lawrence, I recently had to perform some PQRs for 3/16" plate, single side V groove, no backing, CJP for GMAW in 2G/3G. I hope you don't have to try anything like that!