Bi-Amping Possibility, which direction ?

One of my audio systems thinking about biamping.
Have a pair of Pass X600.5 mono amps & Class A Threshold SA/4e stereo amp. Would like to put X600's on the bottom and SA/4e on top for biamping a pair of Sonus Faber Extrema speakers.

I would like to keep the system totally Balance XLR.

My preamp is the Marantz Reference SC-7S1 preamp whichhas (1) Balance Output. But has two Unbalance RCA outputs. I love the Marantz Reference SC-7S1 preamp. But again want to keep Balance configuration.

Should I go out and look for a Balance XLR Passive Crossover ? What direction would you go ? Or how would you set up.

Thank you to all who help.
If you get a xover, make sure you know how to properly use it, or have someone hook it up for you that knows what they're doing. Like anything else in your chain, an improperly used piece of equipment (expensive or otherwise) can and will do more harm to your system than good.
It would also do you well to find out more about bal. vs. unbal. configuration; find out if it's something you should really sweat over.
Nice speakers........a customer has a set.

Lots of dumb questions come to mind, like do you know the order, alignment, and, of course, crossover frequency of the speakers?? Simple passive balanced crossovers can be built. Emphasis on simple. Depending on the type, you may not be able to easily do it passive. In which case, balanced may be a pain to do. Tricky crossovers are best done single-ended, and you convert BAL-SE-BAL with a bunch of active components. Like Apogee did on one of their set-ups.

I think the trick with the Extrema is that it uses a coil to shunt the HF to ground for the tweeter. Could be wrong, but I seem to recall someone in the know telling me what was different about those speakers. But that was years ago.
Look for a balanced active crossover, or have a custom 1 in 2 out balanced cable made. Not sure if preamp will optimally drive the latter configuration.
I'm affraid that you'll only have a chance to play with biamplification with no success or even decrease your system quality...
I would try bi-amping without a crossover first. Use the coax to the threshold and balanced to the pass. Then, switch to balanced for the threshold and coax to the pass. See how it works before you knock it.....then tell us what is lacking. Your system should sound amazing!
Well if lateral is the way your going you have to have one so why not the venrable Brsyston?I think it does not even come with RCA's you need cheater plusgs to make it work.Not sure that other big company Marchand has XLR's but they have sooooo many models/config's I assume it is the case.
I agree with Onhwy61. Try to find a balanced crossover. However, matching output levels can take sometime. Your statement sounds like you have only one balanced out but two unbalanced out. I would try whatever you have two of rather than Jerry rigging.
When I had Magnepans I was all set to finally biamp these speakers. Ultimately the change to Soundlab speakers changed tihs, but the one crossover product that appealed to me was the Space-Tech-Lab tube units. They were available in single-ended and balanced configurations. And with the option to go with a tube regulated power supply, this was a serious step above the Marchand.

Check this out

Albert of Space-Tech-Lab is a very nice guy and willing to help with your specific system needs.

The other model to investigate is the solid state XVR from Pass Labs. This model is configurable like nothing else out there. Unfortunately it is also quite expensive!

Know that Krell used to make units as well and one may come up used.They made cards for popular speakers and one might match up as well.Those units jafox put up were yummy looking but a lot of bread.I can see why most folks will take two amps that have conversion switch and go vertical for twice the power.If you take to equal amps you get 150% of ones rated power.No doubt less stess on each amp (and driver thus bette soundis reason to do this plus some more power.But geting differing amps in sound charachter and rated power changes picture greatly.As one of those who has to keep a limit on spending I often wonder how it can be done without a crossover.A review in in Stereophile of the high powered/value VTL Itl 85 integrated which must have put out prety nice sound with El34's and still had 85 watts.The reviewer and the blurb in the buying guiode said "most audiophiles should consider the integrated and get a matching power amp (same cicuts no pre-amp) and that one should consider ponying up for a pair and for amodest investment have close to a state of the art or a combo that would at least never leave you wanting.But nothing was mentioned with this bright idea of needing a x-over.Wonder if you need it or it depends on the type of crossover in the speaker and this what SEEMS to being said above.So many speakers even ones for $150 have "multiple bi-amp/wiring" posts and you read all the time that this prte or that integrated has vaiable out for bi-amping buit never is it made clear to the neophyte if a corssover is necessary or not.Guess I just need to call some companies to figure it out.A well know reveiwer was kind enough to send me a response to the basic question but unfotunately before i could commit to my porous memory I lost my emails.Hope one of the denizens here can give the basic yet complete skinny on the matter because even at this post maybe it's right infront of me but I am not seeing it.Again understand why bi-amping horizontally would be beneficial but again see why most folks dump piece for larger amp if they need it or if they have mono conversion switch just buy a second of same amp for increased power the benefits of going horizontal being offst by more power and if one needs a crossover the extra cost.Can any one help this hapless pea brain?
Dual binding posts do not guarantee that a speaker is designed for bi-amping. It is possible that these posts are provided for bi-wiring only. You should find out what options Sonus Faber affords you before jumping into this. In some cases a passive crossover will be damaged by the insertion of an active crossover. I remember Michael Kelly warning me against doing that to my 10Ts.
More is not always better. Less can be more. I would use the Passes only, employing the shortest speaker cables you can and whatever length of balanced cable you find necessary.