Mcrelly,
I have to ask about the set-up you used. The WT800 is 800 watts at 4 ohms or 8 ohms bridged depending on the model and the WT550 is 750 watts at 2 ohms... You can hear a difference of 50 watts???????? I originally had an 800A model but sold it and bought a new 550 to reduce weight, etc... Now the main thing the 800 brings to the table is the ability to use the "balance" knob when running 2 dis-similar cabs in dual mono AND a really much better compressor.
As for power, I ran this rig at 2 ohms for 4 years and never had an issue...
There is a HUGE difference in volume and low end reproduction between the WT550 and WT800 run at 4ohms (i.e., the WT550 into a 4ohm cab and the WT800 bridged into an 8 ohm cab).
I would think once you get to comparing 2ohm operation of the WT550 (700 watts or so) with the 4ohm bridged operation of the newer WT800 (1100 watts I believe.. basically the WT800 running 2ohms per side), the difference would be less, since you are at such a high absolute wattage level with each one.
That being said, as posted above, cabinet SPL and tone preference (i.e., tight and punchy versus big and boomy) can easily make 500 watts more than enough or not even close, depending on how loud you play.

