Biamping, yes or no?

I agree with both Csontos and Edwyun. Four identical mono amps and an external XO. You will need to know the factory X-over point and slope of the D1s. Using these parameters as a starting point, with the external XO's. The internal passive X-overs will need to be by-passed completely. As you can see, bi-amping involves more electronics, speaker cables,IC's, power cords, footers, etc. Is it worth it? Only you can answer that. Is there an improvement in sound? Absolutely!

^^^ Yeah, using the speakers' existing crossover point is a good place to start. If you can get more info as to the slope (e.g., 6/12/18/24 dB/Octave), that would also help in testing the settings on the active XO.

Though with steeper slopes, I often get phase changes, which is not a good thing.

Each 'slope' adds 90 degrees of phase chift.....so you ALWAYS get phase changes with a crossover. 6db=90degreees......12db=180degrees and so on.
It is even possible to need to wire the mid/tweet out of phase with the woofer if you use a 12 db slope. That puts 'em back in mechanical phase.

As for SS / Tube for biamping? Other considerations exist. For example....does the sound take the same amount of time to go thru each amp? This will be a real issue at and around crossover where it would be nice if the music was all 'of a piece'......

Another consideration is power to each amp. If the crossover is around 500 to 600hz....you will need equal power above and below the crossover. Higher frequencies of crossover progressively use a smaller percentage of total power needed. Above say....10Khz, you may only need 15% of the total power.

I think 'active' is the way to go. But be sure to use amps with matching gain structure.

Yes, its worth the effort exploring whether you can coax more out of your system. May not work in all systems but sure did in my case. However I am sort of a gear hog and had almost all the additional components to try bi-amping, first passively(using the stock crossovers) and finally moving to active with an electronic crossover.

You should first do some research, start here: http://sound.westhost.com/bi-amp.htm

In my case I started with simple passive biamping, 2 identical amps. Then I tried a tube amp on my mid/hi with a SS amp on the bass. The tube amp had a volume pot which helped with level matching (luckily it was also the more sensitive amp) which is essential when biamping. My speakers had first(on the bass>mids) and second order slopes (on the mid to hi).

WHen I went to an active crossover, I simply kept the mid to hi crossover and removed the bass coil and part of the mid crossover and used a Marchand electronic crossover to split the bass freq. from the mid/hi. You need to know technical details about your speaker crossover, like are there any EQ networks for any drivers, etc...
Sine I am DIYer, I enjoy the hands on experience plus maximizing the performance of the gear I own.

So yes, it can be beneficial, but its takes time and effort and resources(money or otherwise).