Hi giannettino!
I have had the old Rega Planet for at least 6 years, and within a year bought a Theta DSProBasic IIIa DAC to add to it. Maybe old news now, but a real expensive DAC at the time.
I have listened to a lot of other's cd unit setups and gone thru a lot of amps and speakers, but I am still happy with my Rega/Theta combo.
Your mileage may vary of course, but I've never had a good enough reason to change. I think of it occassionally, but then I forget about it and go tinker somewhere else.
So in answer to your question, in my opinion, a decent external DAC would be a nice complement to your original Rega Planet.
Best Regards,
DeadFish!