Ooops I was wrong- the L1290s' woofers are crossed over at 500Hz and below, the L1590s' woofers are crossed over at 350Hz and below (even though both speaks use the same midrange.) ADS did provide this info in the manual- I just didn't look hard enough.
Jhorsh- you cannot biwire these ADS models the way you can biwire a typical speaker with 2 sets of terminals today. Biamping is the only possibility because of the internal crossover circuitry configuration, and to do this you must have some sort of external crossover mechanism (I'll get to that in a sec.)
The right hand set of speaker terminals on the back of your 1590s are for complete full-range driving of the speakers. If you are running each speaker full range off one amp or receiver channel, you connect the speaker wires to this set of terminals only, and the biamp switch (the bottom switch on the back panel) is set to the right for full range operation. However, if you choose to biamp, you set this switch to the left, and connect the low frequency speaker cable to the left set of terminals and the mid/high frequency to the right set. BUT (and here's the problem)- setting that biamp switch to the left deactivates the internal low-pass crossover for the woofers. (The tweeter and mid are still protected by the internal mid and high pass crossovers, so you can run them full range off any amp, but the low frequency signal MUST be crossed over externally somehow.) If you try to biamp or biwire without an external crossover somewhere, you'll end up running the woofers full range, which will not only sound wrong but may damage the drivers.
Back when they made these speakers, ADS also made an optional complementary amp/crossover component called the PA1, that was specifically designed to fit in the back cavity of the 1290 and 1590 speakers (This is what the first post was referring to) that allowed biamping. They also put out an external crossover unit called the C2000 that you could use in conjunction with your own amps. But both these units were only made for a couple of years and are extremely hard to find- so the only option for most of us to get the most out of these speakers is to use external amps and crossovers. If you choose to try this, you shouldn't have too much trouble finding a suitable amp with a low pass crossover setting of 350Hz (where your 1590 woofers are crossed over) but I'm having a heck of a time finding one that will allow a 500Hz crossover point for my 1290s!
To be honest, driving those 1590s full range off that Mac would probably give you about the fullest potential the speakers have to offer!
