Recommended Amplifier Power [W]30 - 180 For the Dali Oberon 7. My setup will be 7.2.4.
https://myhometheater.homestead.com/splcalculator.html
If you use this calculator the Denon 3700 will provide over 100dB SPL with the Dali Oberon7. That's more than is healthy. You could add an amp but the surrounds will be fine on the Denon.
If you want to buy an amp anyway I would consider a high quality stereo amp rather than a 5 channel. My suggestion would be a Purifi based Stereo amp like the March Audio P452 ($1600) or the VTV Purifi ($979).
https://vtvamplifier.com/product/vtv-amplifier-purifi-eigentakt-stereo-amplifier-based-on-eval-1/
The Purifi amp design is high quality and can be used with your front speakers to help provide a powerful system with your Denon 3700. You can also setup one of the Denon Presets with the Audyssey app to be in Preamp mode with "Amp Assign" and just use the Stereo amp for awesome stereo presentation.
Personally, I think a high quality stereo amp with Denon 3700 makes a lot more sense than adding 5 channels externally.