Bose 901 Series ii

Yes, I agree with roberjerman, a pair of Vandy's would be a move upwards. I only mentioned the McCormack's should you want to keep the Bose.
I remember the 901's. They had a nice openness, but, as you said neither great highs or lows. A pair of Vandy 1's and 2w subs would probably blow the Bose away. You'll not only get the nice open sound, but a better soundstage with more definition of instruments and vocals.
I have heard the DCM time windows years ago. They were pretty nice, but I think a current pair of Magneplan speakers would be a better choice as they would be easier to run. (as would the Vandy's).

If you still want to keep the Bose, then perhaps going to the Adcom gfa555 and matching preamp would be another inexpensive way to power them. Though I still think the HK pre is probably your limiting piece of equipment.
B

I would go head and sell those old.. very bad sounding 901's, they are much better speakers out there!..  Used Martin Logan ESL's are much better for the money! Or wait until you can buy some new Martin Logan ESL-X's!

Has anyone ever tried stacking Bose 901s? 

Though almost everyone trashes Bose, I remember listening to my first pair for hours, and they're  some truly great memories.
Bose, to me, has that jukebox bass (warm and rich) that goes surprisingly deep. Nice staging and pleasant voicing. 
Soooo, I'd keep them, buy a nice Integrated as someone suggested. One with a bit of power, because as someone pointed out, they're a bit hungry.
Wyred for sound, makes a good amp.... also consider the LSA Integrated. Used, it's inexpensive and has balls.
Good luck.

PS... be sure to listen to Carley singing 'De Bat Fly in me FACE after buying the new gear.... great fun.

Make that Wyred4Sound.....
oops.