Is a preamp needed ???

It depends. I would encourage you to scan these pages, as there are many threads about this subject already......

I would imagine a quick call to a Lumin dealer would get you an answer.

What amps you’re using....tube or SS. Depending on rest of your system, you may be alright with Lumin T2.  

What other digital sources you’re referring to? T2 will only allow you hear music stored on a external HDD. It does not have any analog inputs you typically found on a preamp. 

In any case, I would bring in the T2 in your system and let your ears guide you. 

Technically speaking there is nothing preventing you from connecting the T2 directly to a power amp.  The T2 has a volume control. 

The question is if the preamp stage that is built into the Lumin is better or worse than an external preamp?  I will preface this by saying that I have not tested a Lumin preamp stage and it will depend on the quality of the preamp you are comparing it too. 

I have tested the preamp stage in a PS Audio Directstream DAC vs. three separate preamps paired with three different power amps.  I could not find a scenario where the preamp in my DAC even came close to a standalone preamp.  In all combinations the soundstage was flatter, less dynamic and noisier than a separate preamp.  It's not that the DAC Preamp stage was bad, just not as good.  

In fairness, the preamps I was comparing to were the Ayre K5xeMP, PS Audio BHK and an Art Audio Conductor so all spectacular units.  

That exercise made me realize how important the preamp is in your system and I view it as a non-compromise component now.