Because the XTRONS head unit outputs its full 25W x 4 amplified output via  its ISO plug connector (just as most aftermarket  head units) YOU SHOULD NOT  connect the XTRONS ISO plug  to the Discovery 2s ISO speaker wiring plug IF your car has the factory amplifier - as the volume will be far too loud even on the lowest volume setting & it will amplify any background noise  a lot. It may also destroy the  LR amplifier if high volumes are used repeatedly !
XTRONS provide a non amplified speaker output via 4 individual RCA (aka phono) plugs which need to be connected to the D2s speaker connection ISO plug to feed the head unit's output to the car's amplifier and then onto the speakers.

Unfortunately,  AFAIK no aftermarket supplier makes a 4 RCA connector to ISO plug adapter lead that I could find - so I ended up making one up.
NB it is possible to  use the XTRONS amplified outputs to connect directly to some of the D2s speakers, but you'd have to modify lots of the vehicles wiring to  bypass the standard fitment amplifier (located under the front passenger seat) and decide how to connect the many speakers (12 on the Highline system I think)  to the head units 4 outputs (FL, FR, RL & RR)

XTRONS and many other aftermarket  suppliers sell "add-on"  DAB / DAB+ radio modules (stick / dongle) that can be added to many  android head units that have a spare USB connection.
The unit should come with  a stick-on aerial for the windscreen & relevant connector lead.

A free  "App" , DAB-Z can be downloaded from the  Google Play Store to  provide an interface on the  head unit  - listing  & managing all of the DAB / DAB+ radio stations receivable in your location & (importantly) showing station signal strength.
The XTRONS  website  has 3 variants. 1 with aerials for both front & rear winsdcreens to allegedly improve reception. I chose this based on positive reviews and a 20% off offer.
XTRONS suggest it's only compatible with XTRONS head units, but some good YouTube info suggests any USB DAB module is compatible with any android based head unit.

Absolutely important when installing the aerial is to  ensure that the earthing strip is well stuck to an adjacent area of metal bodywork that has had the paint scraped off to allow  metal to metal connection. Without this reception can be terrible.

After I connected the module  the spare USB connector I started the the already installed DAB-Z App and the App indicated it was searching, but didn't appear to be making any progress. I tried connecting the module to the other USB connectors, but no change.
I then decided to see if there was an updated version of the App via the Google Store . There was a newer version which I updated to before re-opening the App. This time the App asked various questions about "set-up" before again starting a station search, but this time showing a "Progress bar". After a couple of minutes more than a 100 stations found and the App gave the option to search for and download individual station logos .
The head unit was already connected to our house WiFi network, so I'm not sure if that was required for the logo downloads or whether it would happen "Over The Air" anyway.
After another couple of minutes, I had a large list of stations - many of them showing almost max signal strength and providing great sound quality. 

Driving around, the sound quality for stations with strong signal is good with very minor oddities, but stations with medium to poor signal strength is not great at all and they "stutter" and break-up.
Once working OK, I swopped the module into other USB connectors and after the DAB-Z App re-started, it worked OK with each of the connections.
Overall, I'm impressed it works OK and adds a lot of radio station choice.
FM sound quality maybe very slightly inferior (despite the BS about DAB / DAB+ both are actually digital - so no surprise they're not that different). However FM reception is better because its nowhere near as sensitive to obstructions such as buildings etc.