My experience is that passive biamping does produce an improvement, but it is not huge relative to the cost of buying another amp. This is because you are still using the speaker's crossover. However, if you already have two amps, then it would be worthwhile to give it a try since your speakers have two sets of binding posts. Where you get a huge improvement, however, is when you go to active biamping. But this requires the addition of an electronic crossover and the removal of the speaker's crossover. Stripped of the technical lingo, the improvement is due to the better control of the drivers and the precise separation of the frequencies going to the drivers. The problem is that the speaker designer designed the speaker as an integrated package: cabinet, drivers, crossover, etc. If the speaker wasn't designed to upgraded to active operation, you could get unpredictable results by replacing the designer's crossover with another one. For this reason, I would suggest that if a person were to do this, the crossover they add should try to match the crossover frequency and slope of the original crossover in the speaker.
The poster above who says that you are not biamping does not appear to understand the difference between passive and active biamping. Apparently he has some other issues too that he needs to work out.
I can't easily explain what you are finding if you have two identical amps that are working properly. You have no difference when vertically biamping so this tells me that the amps are probably fine. But the difference does emerge when you horizontally biamp. Because of this, I'm inclined to think that there is something going on in the speaker's crossover. Perhaps these speakers aren't meant to be biamped because of whatever is going on in the crossover. The double binding posts may be for bi-wiring, but not necessarily bi-amping, though that seems a bit odd. I don't know. Perhaps you might want to contact Snell to see what they have to say about having their speaker's biamped. You also might try reposting a thread with a title containing something like: "Bi-Amping Snell speakers". This might attract some posts specifically from other Snell owners who have done this.
