Passively bi-amping

I think you are definitely headed in the right direction by vertically bi-amping. I don't know what your system configuration is but if you can get your amps close to the speakers and use as short a speaker cable as possible will also show additional benefit. Almost as much improvement as going to bi-amping.

I also agree with the others about perhaps needing some more juice. I would be worried about the bottom end potentially clipping. Beyond that, there is a definite sonic improvement when your amps have lots of headroom and current to spare. I bet if you where to get a pair of 250.5's you would be quite surprised at the improvement in depth and clarity. That's not to say there is anything wrong with your Alephs, but just think what they could sound like if they were only driving the mids and highs. Your B&W's are similar to mine and Beemer's Aerials and we know how much these can open up with the addition of more quality power.

Another thing you could try is to keep the Alephs on the tops and get a more powerful Pass amp to run the bottom end. Yea, I know, I'm just spending your money. :)

If you go active, you can get some monster power(Sunfire 600 for example) for the bass and leave Aleph to do the rest of job.

B&W X-overs even on flagship models is not the best you could think of. Adding an electronic X-over(bypassing or removing the built-in one) will give you more flexibility to get from your speakers the best jewels without breaking the bank on a pair of x250's.

I hear active bi-amping is tricky and my personaly opinion is intergating subwoofers is almost impossible to get right.Plus my room is bass heavy. Could use a pair of 16" Tubetraps. I have a small room for such a large speaker and am able to acheive sufficent volume levels with the Aleph's. I don't have a lot of headroom. Turning the 802's any louder than my normal rock n' roll levels does cause bass clipping. Just have to keep an eye on things. I'm running 8ft Transparent speaker cables. Just wish I could have turned those 5's into monoblocks. Nelson Pass said no way. Don't have the scratch to be investing in any more equipment right now. Just be happy with what I have... Thanks for the feedback!

I do have some respect that these a under powered amps (Pass Aleph 5's) for the Nuatalus 802. I have a notch in my Aleph P. When turned all the way to it is plenty loud for this rock and roll animal. Bass is tight, no sign of distortion. However if I turn it up any more my amps run out of gas and the bass begins to clip. So I keep it under the notch and the sound is fantastic. Steely Dan's Aja is mind blowing. So is Gish by the Smashing Pumpkins. BTW I'm an analog enthusiest and also have a Pass Aleph Ono. Will take the Pepsi Challange against any CD player.