Your pre and power amps are fine, and won't likely be bested by very much out there now because in terms of linear amplifiers, there hasn't been much if any change or new technology in years. The Vandersteens are not my personal cup of tea, but are fine and if you have no issues with their performance, why change unless you have the overwhelming desire to spend money?
Likewise the digital gear is all good and any 'improvement' by substituting a different unit is likely to be largely imaginary.
To add HT functionality, there are two ways you can go about it.
1: Run everything through the Denon and not use the Rotel pre. Use you Bryston by sending signal to it from the Denon pre out (assuming it has them) for L/R. Surrounds will be powered by the internal Denon amps, and all levels set by the Denons calibration features.
2: Run all H/T sources as described in (1). If you wanted, you could still go 'purist' and run the Cambridge analogue outs to the Rotel pre, and when only using 2ch sources either unplug the Bryston from the Denon, and use the Rotel as pre, or use a selector box.
I doubt the difference in 2ch will be all that dramatic between 1 and 2 but may help your 'inner audiophile' come to accept the inhumanity of having HT gear present in system.