Sounds like you need an active XO. check out Behringer DCX2496 mods from Scott Endler. YOu will need to be able to bypass (or defeat) your speaker XOs.
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(http://www.tactlab.com/Products/22MINI/index.html) Check out this unit. I am presently using the big brother RCS 2.2X and have been very well pleased by it's lack of any negative audible impact. I've been familiar with the professional digital crossovers and signal processors for years being a sound tech for a number of local venues. The manufacturers have to use less expensive internal components to keep the cost of their units in the $200 to $400 range. They WILL leave a sonic footprint. TacT Audio won a 'Golden Ear Award' from The Absolute Sound in 2007.
If you're using the GFP in passive mode, it's the source that would be the problem.
When using identical amps in parallel, the combined input impedance is reduced by half. The GFP has an output impedance of 600 (RCA) and 1200 (XLR) which, by the numbers, should not have a problem driving 25K input impedance. The 333's have twice the impedance for XLR vs RCA and probably more gain as well, so you got me stumped how that works decently. Combining RCA and XLR can mess with the CMMR (noise reduction) of the circuit.
I've successfully biamped with much lesser and more diverse gear than yours but it hasn't always been easy. Currently, I have 3 amps (nearly 3 KW combined) with a passive TVC, so anything is possible.
Inserting an active Xover is more complicated than it may seem and will require technical knowledge as well as possible surgery to the speakers but that is the ultimate.
I know the 801's are power pigs but do they really need dual 333's. I'm not one to talk and I'll agree that most systems are underpowered, but is it really worth YOUR trouble?
Another possible option is bridging the 333's and using them for each channel. That can result in 4 times the wattage but not all amps can be bridged successfully, espescially into low impedance speakers. Contact Levinson if that's advisable.