I found the owner's manuals online: the 29's at marklevinson.com and the 20.6's at hifiengine.com. Gotta love the internet.
Comparing the specifications for both amps, the 20.6 has 26.9dB gain for 40V rated output. The 29 has 26dB for 30V output. The input sensitivities aren't given so I calculated them based on this information. Unfortunately, they aren't the same. Scaling both to a 30V output, the 29's is 1.5V and the 20.6's is 1.35V. This may not sound like a big difference, but in my experience with horizontal biamplification (which I currently do in my system), it will definately be audible.
However, there is a chance that the speakers he is using would have a "balancing" resistor between the lower and upper crossovers that "could" go some distance to alleviate the problem since it will be effectively removed when he takes the jumper straps off his biwire posts. The sensitivies aren't too far off so he should try both amplifier combinations and see if either of them works with his speakers, room, and tastes. Who knows? He might get lucky.
It would be preferable to have the higher-powered amp on the bass but the opposite experiement won't cost anything to try. If both fail, the only option (short of changing amps) is to use an external crossover to match the gains but that represents an extra pair interconnects and a potentially costly piece of gear....
Sound quality-wise, I personally feel that (again, in my personal experience) the amplifier "voicing problem" some people refer to in horizontal biamping is swamped out by the different driver configurations that the individual amplifiers see. In other words, damaging the sound quality by horizontal biamping is rare because bass drivers can't sound like midranges and tweeters in any case. It simply boils down to an EQ effect, either beneficial or detrimental depending on the circumstances. This is assuming the bass drivers are truly bass drivers and prove it by having a suitably low crossover point, such as 200Hz.
But a key point to make here is that the extra head room and power his amplifiers would gain would go a long way to making the soundstage bigger, dynamic range larger, and fine gradations kept intact. So if anything, he should get an improvement by horizontally biamping. Of course, this is also assuming he can get perfect gain agreement between both amplifiers.
Arthur
