I always liked the Bose 901's (and it is probably that the competition has gotten better), but my impressions is that the new 901's are not as good as the old ones. Maybe it is the plastic and light weight, but I feel they have cost-cut on the 901's in ways that impact the sound quality.
I don't know where your series 5 fall in the spectrum of "modernization.
Edit: I read that they were originally a sealed design and starting with the series 3, they ported the 901's which forfeited much of the bottom octave but greatly reduced the need for high wattage. That fits my experience as I remember the 901's I listened to in HS (1975-6) having really nice bass and when I visited a Bose Store a couple of decades later they were a different animal.
Regardless, the most obvious thing (IMHO) about the Bose 901's is they have always had the full soundstage of an omnidirectional speaker. This makes non-directional sounds such as the strings of a symphony sound pretty realistic. Conversely, a trumpet (which is highly directional due to the bell/horn) will never sound quite right.
For me, this is neither good nor bad, just a trade-off to recognize. In this context, if I listened only to trumpet, 901's would suck and if I listened only to cello, the 901's would rock!
The next most obvious thing about the 901's is poor treble extension. Those 4" full range drivers just can't compete with a dedicated tweeter. A common modification of the 901's was to add a "super tweeter". The Radio Shack Realistic super tweeter was a popular model for this, but, unfortunately it was more than a simple matter of "plug and play".
And third was the poor bass response (after series 3). In today's world adding a subwoofer is the most obvious way to deal with this.
In a Bose store, you will notice the music they select to play on the 901's (and probably throughout the store) is mostly midrange with very little content at either the high or low frequencies.
The 901's do okay on cymbals, but not if you had the real thing nearby. A triangle will further reveal their lacking and symphonic chimes will be severely lacking in overtones:
If you have a decent set of headphones, compare what you hear (starting at 7 seconds) on them to what you hear over the 901's using the link below:
Understand this is an extremely demanding sound for a tweeter to really get right, so many good speakers will not compete with good headphones, but your 901's will be obviously poor at it.
When the time comes, I would recommend doing a trial with the SVS Ultra Bookshelf speakers. SVS gives you 45 days in-home trial with free return shipping if you decide not to keep them.
https://www.svsound.com/products/ultra-bookshelf
Being able to listen to them in your home for weeks is really ideal for testing speakers. In your situation, you may have a hard time dealing with the directional sound of a typical speaker after having listened to a reflective speaker for so long. If you aren't willing to give up the reflective sound, but recognize the lack of treble, I would recommend you look for Mirage Omni speaker on Craigslist or ebay. There are several different models, but this link will give you a basic orientation to their design:
https://www.soundandvision.com/content/mirage-omni-250-surround-speaker-system-measurements
Also check the third page of the above link for a discussion of the merits of omni-directional sound!
There are probably some omini directional speakers currently being produced, but I do not know what they are.
