@jonicurn
There was something that attached you to these speakers at some point. Only YOU know what that is. We service vintage audio gear and have had many 901s and/or associated gear come into our shop. Typically the owners are VERY happy with their 901s. If I were to offer to substitute a highly regarded "audiophile" speaker worth many times to price of their 901s, I’d get flatly rejected.
I agree with @stringreen ’s suggestion about property set up. It would also be good to know what sources you are listening to.
The description of "flat" sound can mean a lot of things. In my world "flat" would mean "lifeless". The best "audiophile description" I could offer would be a lack of dynamic contrast. We want the percussion to sound "live" when the drumsticks meets the sheepskins, the lead guitar to be pushed forward, and the vocals to be up close and personal. There are many elements in the music chain that can squish dynamics. Having a high watts amp/receiver is helpful, but "loud" is just "loud" if dynamics are compressed. Modern low-priced receivers don’t have alot going for them in this department. They force all the signals thru DSP processing (even when you are listening "flat") and "chip amps" designed for low cost are not noted for their musicality. So, I’d start with a good totally analog receiver/amp. The more power the better, but I’d inject that the right 50wpc receiver will sound better than the wrong 100wpc receiver. Your sources, cabling, etc also matter.
Only then will you know if the sound of 901s is still appealing to you.