Hi Bill
I had a visit from a guy yesterday who owns JR149's (I'm sure you remember those if you were on the hi-fi scene in the early 70's), which are esepsentially your HF and mid unit in a smaller box. He'd come in to audition the KEF R500 floorstanders, and he was extremely impressed.
If you're after a standmount speaker to replace your 104ab's, give the KEF R300's a try - a £1,000 3-way standmount that gives floorstanders plenty of competition as far as bass extension is concerned, and is pretty much the best at this price point for midrange and treble clarity. Good stand are needed though as they're pretty heavy. We had them running in in our demo room for weeks, and we never heard one single negative comment about them. One of our customers who runs ATC actives couldn't believe how good they sounded for £1k.
Twenty years ago, there were plenty of top notch sub £1,000 floorstanders on the market, but today, it's a different story. Because of rising costs, things have changed drastically. There's the KEF Q700 (from the range below the R series already mentioned), which will improve in certain areas over your 104ab's, but you might find certain aspects of their bass performance lacking a little compared to yours. Personally, I'd be recommending the R500's at £1,500.
The best thing to do is go and audition a few to get an initial feel, and preferably take your speakers. Its hard to recommend a few alternatives without knowing your sound preferences, and what aspects of your current sound you like. Some people with speakers from the 104 era like a very full, prominent bass with a very smooth, laid back treble and midrange. Speakers have changed drastically. The sound ive just described would now be classed as overblown and veiled, and quite boxy. Generally, they now have a more forward sound that is a bit of a culture shock for those with older speakers. But this is the price you have to pay for a more revealing, informative sound that gets closer to the original.