Bose 901 Series ii

I strongly recommend you think about a good integrated then. How about Parasound? Lots of inputs, a very nice built-in DAC, and plenty of power.

Keep the electronics and ditch the Bose's! Buy a good used pair of DCM Time Windows and experience truly great sound! I am presently listening to a pair (serial no's 52779 and 52780) driven by a GAS Thoebe preamp and a Son of Ampzilla (both circa 1978). The TW's were bought on EBAY for $219 + shipping (in excellent condition!). The biggest (and best!) improvement for you would be a speaker upgrade. If not TW's, then I recommend Vandersteen 2's.

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?