Did you base your custom 752 on the Original 752, or the upgraded 'Freedom' model? The 752, and 753 freedoms were highly rated speakers (I have 753 Freedoms myself). I believe both the crossovers, and tweeters were reworked in the freedom models, plus additional cabinet changes in the 753's.
If your mods improved the stock sounds, I very much doubt that a £600 new speaker will improve it much. Hifi diminishing returns apply, and I would have thought you would be looking at a grand, to get a real improvement on a speaker that even in 'stock form' would have been around £600 new.
Of course, speakers often have a 'family' sound. If your simply 'bored' with the mission sound, then the best advice I can suggest is visit a small 'professional' hifi shop, and ask for some demo's.
Personally, I find that my 753's only really sing, with a powerfull amp driving them. Cyrus One amps, are good, but only 30WPC, with later Cyrus amps being rated at 50WPC.
Sure new speakers may be the thing for you, but I would certainly check out some different electronics before splashing out on new speakers.
Whatever you decide, try before you buy!