Your guesses at ohms load are off. Basically you can run both of the 8Ω 115 cabs AND the 5.3Ω cab all at the same time. That would generate an ohms load of 2.28Ω. So you can run all three cabs at once from your WT550 amp as it can run a 2Ω load with not problem. The acoustic 612 cab will get a bit more power to it than the 115's will because of it's lower impedance, but should be pretty close.
There were actually two versions (A & B) of the WT550. The A version puts out 300-watts into 8Ω, 500-watts into 4Ω and 620-watts into 2Ω. The B version puts out 300-watts into 8Ω, 500-watts into 4Ω and 750-watts into 2Ω. The B version came out in 2005. I've attached a copy of the 2003 manual for your reference. The manuals were the same, only the power rating on the B was a little higher than on the A version. Thought I had the 2005 manual, but couldn't find it...hope it didn't get deleted. As long as I was attaching manuals, I included one for your WT1205 too just in case you don't have it.
A better choice would be to use your WT1205 specifically because it is a dual-powerblock amp. It's almost the same as having two WT550's in one amp - one for the left channel and one for the right channel. It produces 325-watt rms into 8Ω per channel, 500-watts into 4Ω per channel, or 600-watts into 2Ω per channel. It can also be bridged into one cab with 1,000 watts into 8Ω or 1200-watts into 4 Ω (can be a couple of 8Ω cabs).
I would suggest you plug both 115's into one side and the 612 into the other. Then you can use what I refer to as the "Magic knob", which is really the "Balance" control on the front of the amp to adjust the blend of the cabs in the right channel with the cab in the left channel. Just turn that control it the direction of the side that you want to be louder. Don't make huge adjustments, make small adjustments until you get the blend of sound you like best. If running in bridged mode, center the Balance control at 12 o'clock.
I used to do this Balance knob with my Eden WT500/800 when I ran an acoustic B410 on one side and a GK115bpx cab on the other side. That GK cab was really a boomy cab, but with it plugged into one side all I did was adjust the balance towards the 410 side until the mud went away. Previously I'd run the two cabs through an Eden Nemesis RS400 and had to drop the Bass EQ to about 10 or 10:30 to get rid of the boomy muddy sound of that GK and that poor acoustic 410 really did need the Bass EQ back to flat to sound good. So when I ran it through WT500/800 with one cab on each side, I could run a little less power to the 115 and that cured the mud without cutting the bass EQ.
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seriously though, it sounds like your speakers are not tailored to the low frequencies you’re after. The BE are loud but have a pretty steep roll off around 46hz. They make bigger cabs with lower frequency response - but they are not an easy schlep. You might want to consider another maker - there are plenty out there who can help you find what you’re after.
