My suggestion is to read all the older threads ( spread throughout this great site ) about the 4345 ( & bookmark them all / if you haven't already ).
- They do offer a very good road-map in moving forward ( quality > listening wise, IMO ).
The standard crossovers ( mentioned in those threads ) are all quite acceptable electrically ( from this guys perspective ).
- The analogue types mentioned, were models from Bryston & Ashly ( & to a much lesser extent , the ancient JBL 5235 variants ).
PS; If you decide to put a separate amp on the UHF, I believe GT was talking about a tiny classD GainClone of sorts ( preceded by its own dedicated HiPass ). This could easily be something from miniDSP ( out of China ).