How old is too old ?....

Upgrading speakers will add detail and linearity, but...

You still may prefer the 104's. British speakers of that era captured a beauty offered by music that modern speakers with all their speed and "accuracy" have difficult reproducing.

Upgrade as an experiment but don't be too quick to sell the 104's.

Art

Since you are up here in the Great White North, I would wholeheartedly recommend Canadian made speakers. There is a variety at different price points. A good number of which leave nothing to be desired. Energy Veritas 2.3s, Paradigm Studio 100 v2s (or are they up to v3 yet), Gersham, PSBs, Mirage, Verity.

You will be amazed at how much performance you can get for the dollar these days, especially if you compare older foreign designs to newer Canadian designs. Keep Canada green, buy Canadian products. Listen globally, buy locally,

Good day.

16yrs is really way too old. Go buy new ones immediately. There, that's what you wanted to hear.
If I were you I'd listen to some new B&W and Kef reference (amongst others) and decide whether it's worth it.

I have this exact same dilemma. I have upgraded my electronics, and then wondered if the KEF 104.2s were worth upgrading. Luckily my friend had a pair of new B&W CMD7 NT. These are the same price as the 104's were back in the early '90s, so with inflation the B&W are actually cheaper today. The reason for using these speakers is that I thought they were in the "same league" as the 104.2 in their day. Anyway, we brought the speakers over, spent about an hour dialing in their location in my room with my electronics, and the results surprised me. The stage, the depth, the accuracy, and overall reality of the presentation was far better in the B&W’s. As a long time fan of these speakers, I didn’t want to believe it, but it was true. The technology has gotten better, no doubt. The high quality electronics that you own, you are not even hearing yet, trust me. Take you 104’s and may use them as a start to a surround sound system, and get a new pair of speakers, so you can finally hear your “system”.

I have the KEF 107s and just, not too long ago upgraded my front end (CD Player and Pre Amp.) After upgrading the front end I went out a picked up the Dynaudio 1.3SE and the difference is night and day, don’t get me wrong the KEF 107 do have the deeper bass, but that where it ends. To me the Dyns are just a better speaker . the highs are sweeter, the soundstage is wider and deeper, the bass also is still very good for a monitor speaker. Up grade your speakers you will feel go about it.