I have found that a multidirectional array of poly tweeters will easily outperform any single front firing tweeter. The reason is that it is the only way to make the polar response at high frequencies comparable to the rest of the speaker system and threfore the high frequency energy as a function of time is also comparable. This is due to early reflections (echoes) from the direction of the speaker which are unavoidable. This beats pulling the speakers away from the wall or using sound absorbing material on the walls behind the speaker. You also get the advantage of being able to place the speaker against the front wall taking advantage of the bass reinforcement this offers. All of the sound systems in my house used for listening to music use multidirectional tweeter arrays. In my experience, unless you get the high frequency response right in this way, it is impossible to accurately balance the tone of the rest of the system. Even so it takes time and patience depending on how particular you are.

The Crown CE-1000 offers 275 wpc into 8 ohms (one pair of Bose 901s) and 450 wpc into 4 ohms (two pairs in parallel.) It costs only $450 from Parts Express including shipping. It is the ideal solution for powering these speakers.

http://www.partsexpress.com/webpage....WebPage_ID=133

For the original Bose 901, I used four tweeters per channel each crossed over with a single 2.7 mfd capacitor for a 6 db per octave filter. A second amplifier is used to power the tweeters. It derives the signal from the main preamp output which is unequalized by the Bose equalizer but equalized by a 10 band eqalizer in the preamp and controlled by the preamp volume control. The volume control on the second amplifier sets the relative level of the outboard tweeters. That amplifier has its treble level set about flat and its bass control at full cut. The main equalizer has a cut to eliminate the 901's peak in the 500 hz range, some additional bass boost and a slight high end rolloff.

The front tweeter is hung with two small plastic L brackets from the underside of the top plate just above the front 4 inch driver. Two rear tweeters are screwed directly into the rear edge of the top plate on each side between the 4 inch drivers and the last one is at the apex held into the top plate with some long screws. This avoids drilling any holes into the enclosure which might result in an air leak.

The speakers are installed in a 14 x 14 sun room whose walls are about half glass about 12 inches from one wall and about 2 feet from each side.

The overall result is a very wide range clear accurate sound which preserves the unmachable attributes of the direct/reflecting nature of this unique loudspeaker system. The effect of the direct reflecting sound propagation is what IMO attracts so many buyers despite the other limitations and high cost of Bose 901. I think my pair have overcome the main objection audiophiles have of it, namely, its unacceptable frequency response distortions. I just might have the best sounding Bose 901s in the world.