Now this is interesting. Without loking up my HiFi Choices from this era, I always though that both Ditton 15 varients had HF1300 tweeters as standard...... These either never did, or they've been replaced!
I remember that the 15XR sounded veiled, nasal and "pinched" and with a "boppy" bass compared to the mid seventies competition like KEF Chorales, B&W DM4's, Monitor Audio MA7's and 5's and the original and replacement AR equivalents, let alone Celestion's own UL6's. The Ditton 15's really were at the end of their road by this time, as things had improved so much at this price level by 1975/6.
To be honest, the best Dittons today by miles were the 66's and the replacement one (606?), which still sound good with not too much colouration but restricted maximum volume.