Passive Biamping?

Biamping virgin, never tried it before. Speakers are bi/tri-wire capable, no outboard crossover needed or wanted.

Have four channels of power amplification (pair of monoblocks plus stereo amp, all from same maker and of similar power, if perhaps not exact identical gain), of fully-balanced/bridged design, utilizing balanced input transformers with XLR and RCA inputs.

Preamp is fully-balanced, but has (like many such beasts that provide RCA inputs in addition to balanced XLR inputs) one pair of XLR main outputs and one pair of RCA main outputs.

If the preamp had two pairs of XLR outputs, this would seem easy. Likewise if the amps had daisy-chain line outputs. But alas, like most, they don't. Yet I would like to A) keep the signal fully-balanced all around, and B) not incur the 6dB gain difference that would result from taking both the balanced and single-ended feeds from the preamp. (I've inquired with the preamp maker; unfortunately the RCA outputs cannot be converted to a second set of balanced.)

The preamp (like the amps) is solid-state, and doesn't lack for sufficient output drive/low output impedance, but the input-tranny configuration of the power amps does mean they have a somewhat lowish input impedance at 10K-Ohms, should that imply anything for splitting the feed.

So what is the best path to biamping success here? (No, replacing the preamp is not a good option.) It seems to me this general situation -- biwire speakers and four channels of suitable power amplification all on hand, but preamp lacking two pairs of identical main outputs -- couldn't be all that uncommon, but I've never seen XLR Y-interconnects for sale, if that's what's needed...
That's what is needed. They are available from various sources ranging from musical instrument shops to Cardas.

I am currently using Monster XLR Y adaptors for my biamp setup; got them from amazon:
Behringer CX 3410 balanced model from Musicians Frined <$200!

I use this with biamped Magnepan 3.5Rs
Thanks guys. I have the matched sets of balanced interconnects on hand, so the Y-adaptor route seems like a good inexpensive way to try this out (before possibly investing in custom cables of my preferred type to eliminate the adaptors).

Any thoughts on the best approach to take if I find that a small top-to-bottom gain imbalance between the amps (I'm guessing maybe at most around 1-2 dB) is intolerable? Reversing the amps to find the least objectionable imbalance in my room probably won't be an option, as I presume that their respective sonic strengths will dictate that the mono's (more refinement/purity, but with a little less power) go on top and the stereo amp (more grunt/control) go on bottom.
You may need attentuators on one of the amps. Far less than a 1 db imbalance can be enough to skew the tonal response. You could use a single amp with jumpers on the speakers as a reference point for the "normal" tonal balance. On some speakers, the gauge of the jumpers relative to the main speaker cable may also effect the tonal balance.

If there is an amp level mismatch, you also might experiment with the mixed preamp outputs. Connect the XLRs to the amp with less gain.

With attentuators, you can dial it in. Depending on the crossover frequency, attentuators on the bass amp might not effect the perceived signal purity too much. On the treble amp, YMMV.

Even when the tonal balance is correct, a big disparity in dynamic range or slew rate between the amps may make for a disjointed sound. Again, using a single amp with jumpers will provide a point of reference.
I am familiar (as you might hope!) with the sound of the speakers strapped with jumpers and driven by a single amp (either the mono's or the stereo amp in question here). And like I said at the top, the amps are by the same maker and similar, sonically and in circuitry.

My impetus for wanting to biamp in the first place -- after finding myself, relatively unplanned (thanks to upgraditus), with four channels of compatible amplification on hand, and having lately acquired biwire speakers (my previous speakers were all single-wire only) -- is some sessions of one-speaker mono comparisons I did using the stereo amp to vertically biamp a single speaker, with mono source material of course. (OK, so I fibbed a little when I said I'd never biamped before, but never in stereo.)

Even in mono (with the comparison being against leaving one channel of the amp unused when jumpering the single speaker), the improvements in dynamic contrast, spatial clarity, freedom from congestion, and authority/definition when biamping proved worthwhile. And this is with an amp rated at 500wpc, in a system where the speakers and the room are merely mid-sized, so putative lack of power was never an issue. (I came to the conclusion -- once again -- that, all other things being equal, you can never have too much power.) And so here I am, not selling the stereo amp to help pay for the mono's as I told myself I would.

I do think the 'bass' amp (the stereo one) is the slightly hotter of the two gain-wise, so as you say, padding it down shouldn't noticeably affect the overall sonic purity (the woofers are crossed-over at 350Hz). But I'm wondering how this could best be done in-line, after the Y-adaptor from the preamp. Appropriate resistors across the amp inputs?
But I'm wondering how this could best be done in-line, after the Y-adaptor from the preamp. Appropriate resistors across the amp inputs?
I would leave it as is -- considering the woof amp's only "slightly hotter". As to the voltage divider circuit -- look here Regards.
I agree with Gregm. It is going to be much easier to install an L-pad at or near the crossover for the woofer. The best place would be immediately after the frequency selecting circuit and before any Zobel, if used. That way you can use the driver's nominal impedance and calculate the values for 1, 2 or 3 dB of attenuation. Or, buy a cheap l-pad from Madisound or Parts Connexion. You'll have to evaluate by ear unless you have the tools to do frequency response measurements at the drivers.
I went by Guitar Center today but the only XLR Y-adaptors they had were cheap (as opposed to inexpensive, which they weren't particularly), so I'll probably look at ordering something better online instead.

We'll see what happens when I get the splitters and let 'er rip, but my assumption is that I'll want to pad the bass even if it's just a little hot, because already the speakers are often on the edge of sounding more powerful in this room down low than I would ideally prefer. (I suspect that biamping, absent any introduced top-to-bottom gain imbalance, will probably tend toward alleviating this just by virtue of tightening things up a touch.)

I'd assumed it might be preferrable to attenuate at line level, if possible, rather than at speaker level, on grounds of sonic transparency and parts cost, but maybe that's wrong? But as for ease of installation, I don't think I would attempt the mod myself in either case, not being much of a techie or a solder-jockey. (And I have no way to measure complex impedances at the driver terminals to make calculations.)

I happen to live close by to the manufacturer of the amps, and maybe the possibility exists for me to have them modify the input sensitivity if needed. (I'm guessing that any added or changed resistors would follow the input trannys in the circuit and therefore wouldn't simply be done across the input jacks.)

My preference would be to not modify the speakers internally, so that comparative amp swaps could be made freely, which would rule out padding between the crossover and the woofers. I suppose an inexpensive external L-pad would be a good way to try things out for starters.
Update: I experimented biamping with pro-audio Y-cables ordered online, but these weren't transparent enough to keep in the system. I hated the thought of compromising my reference interconnects.

Then I saw this thread and learned about the hardwired XLR splitter-adapters made by Purist Audio Design. A truly "sui generis" product that has finally solved this problem for me. Not cheap, but worth it in a high-resolution system. Just give them some time to burn-in and they disappear sonically, let you keep using your current cables. Email Purist for details and photo if interested (this item wasn't listed on their website when I ordered).

A must-have for Levinson 38xx-series owners wanting to passively biamp in balanced mode!