Bose 901 VI flat?

I own 1968 Series 1's. Even if the OP has Series 6's which don't I have experience with. Just get a minDSP from Amazon for $125 (100% money back) and dial in the curve for the Series 5 or 6. If the OP likes what he hears. Awesome !

I run Klipsch Cornwall IV's with a 500WPC GaN FET Amplifier and swap in the 901's on occasion for fun. They are great Speakers for what they do. No, you can't hear the fart of a Nat like I can with the CW4's. But I still enjoy what they. No foam rot either cause the Series 1's and 2's (which I also own) are cloth doped surrounds. If the OP's Series 6's are the V2 with the "rolly" surrounds. Foam rot shouldn't be a problem.

To parrot others, you must use the Bose EQ.  I purchased a used pair of Series 1 901s in 1972 because the owner said they sounded flat.  He wasn’t using the EQ.  At the time, I used a Pioneer SPEC-4 power amp, SoundCraftsman Pre Amp/EQ, and dbx 3bx range expander to tweak the music. I enjoyed that combination until about 2018 when I gave it all away to my goddaughter.  

I thoroughly enjoyed the 901s.

Very important to use the equalizers......make sure the V-shape faces the wall..the flat facing the listener. Position them 3 or 4 feet away from the front wall.

All responses to use the dedicated Bose EQ are primary. Also, to incorporate the EQ into the existing electronics is the first step. Since these speakers aren't capable of critical music listening to average ears, everyone has their own priority as to what they want.

Had a friend who had a very large house and entertained often. Had a large room that he used for entertaining guests at parties and such. In that room he had a pair of 901s suspended from the ceiling with chains. Purpose was to provide background music for his guests. Worked well for that. Of course he had his serious system elsewhere.

All this stuff is complex and the end game is what suits the user. Some like  what they have and others always want to constantly improve  theirs no matter how good it already is. Kinda like a hobby.

@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.