Lee, you have a good budget there, and where you spend it depends on where your priorities lie.
eg. How much time do you spend listening to 2 channel?
Your amp can handle better speakers....does it have the option to biamp (using rear channels) and does it have a "stereo direct" setting where all processing is by-passed?
What speaker cables and i/c are you using? Have you tried after market mains cables?
If you want to upgrade your front speakers, you need to be careful not to upset the balance with the centre one.
If the Denon has pre-outs, you can connect in another amp. If you connect a power amp, you are still using the preamp of the Denon, which is likely to compromise the sound.
If going this route, I would look at an integrated amp with an HT bypass (eg. MF M3i), which means for 2 channel, the signal isn't going through both preamps.
So to sum up. I think you should upgrade the front speakers, and maximize the sound of the Denon with good cables (s/c, i/c and mains), biamping and using stereo direct (if this is poss).
At the end of this and the sound isn't quite right, go for a good integrated amp with HT bypass.
You can of course do it the other way round and borrow a good integrated amp. Then just stick it into your system and see what it sounds like. I suspect the speakers would still be the weak link.