Passive Biamping?

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?

Zaike-

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.