Using a Y-Cable on An HD150 head?

Hello there!
Wiring/Load/Ohms/Wattage question (whew, that's a wide range...):
I currently have an HD150 head running a 412GS cab and a 212HD cab at 8 ohms. I just purchased a great set of vintage Ace Tone 4x10 Bass towers rated at 8 ohms and 300 watts each. I know (relatively) that setting the head to 8 ohms will run them and register the load at 16 ohms if I connect them directly to the HD150's series jacks (one wire from each jack on the head to the single jack on the cabs). Correct me there if I am wrong.
My real question is thus: If I use a Y-cable coming out of one of the jacks on the head and run both cabs to that jack, will it register in parallel for a total load of 8 ohms orrrrr something else? I want the load to match up as closely as possible to get the most power out of the rig.
Secondly, is the wattage of these cabs (300 watts EACH, for a total of 600 watts together) too much for the head?
I appreciate all of ya'll's help, assistance and wisdom, as always.
Thank you and let the info rain down upon me!

Yep, run the two Ace Tone cabs in parallel out of one of the HD150 output jacks. That will give you a total load of 4 ohms, so set the impedance selector on the amp to that.
The cabinets are rated at 300 watts each - in other words, you could in theory feed 600 watts in total to the pair without hurting them (would possibly hurt your ears, kidneys and neighbour relations though...). Running the two cabinets as desribed above would provide a maximum of 75 watts to each cabinet. Even running a single cabinet with your HD150 head will be safe at all times.
Best of luck,
Lars Verholtwww.captain-foldback.com

Always a pleasure Lars!
So, just for my future reference sake, using a run-of-the-mill mono 2-female-into-1-male 1/4" Y-cable puts the cabs in parallel?
As for ears, kidneys and neighbor relations, I am trying to get as loud as possible without damaging my good friend the HD150 and it's new pals, the Ace-Tone twins.