Help and opinions needed on pre-amp situation

Trial and error is the only way for you to know for sure what you like best. I know audiophiles that swear by the use of preamps and others that say a direct hook-up is best.

It only took me 50+ years and who knows how many dollars to arrive at a system that I am 100% happy with. The one truism about our hobby is that it is not static (pun intended). We are constantly questing for that next thing that will elevate our sound to the dream we have in our minds. Although I have owned some excellent integrated amps, I have always tended to like component setups because they give you more flexibility to experiment with how your system sounds. A preamp that checks all the right boxes for your preferences is a must have in that regard. Finding it gives you a good solid base to work with, and it can easily become the one component that remains the same in your system. With few exceptions, a neutral amplifier can only sound as good as the preamplifier feeding it.

I would highly recommend trying a preamp in your chain to see if it makes a difference.  Buy a used Benchmark LA4, because it is relatively inexpensive and you can resell it very easily.  More importantly it is a known commodity - super transparent and neutral. I believe you will hear a meaningful difference (not because it is neutral only but because it is a very good preamp and will tell you if preamps make a difference).  Then, if you want to roll in a different preamp you can, and will have a frame of reference.  I have had Lumin Streamers, with Leedh, and never understood it - just sounds boring to me.

A great preamp will add the musical richness to a system 99% of the time. The systems that sound better without are a real rarity. In the rare occasions that they are not required, I suspect they belong to folks that appreciate detail over musicality. Because without a good pream the sound tends to sound highly detailed and a little dead.

@veerossi To add the phono section to the MP-3 is inexpensive if its ordered that way.

Regarding sound quality there are a number of variables. The most common thing you run into is that most of the balanced line equipment made for high end audio does not support the balanced standard even though it might really be balanced.

As best I can make out, the Lumin is an example; I say this because the balanced output has twice the voltage of the single-ended; if it supported the standard the voltage would be the same. One of the goals of the balanced line system is to eliminate ground loops and this is done by ignoring ground in the balanced connection insofar as the signal is concerned. The RCA connection does not ignore ground; when you see the voltage double what it usually means is that the RCA connection is one of the two XLR outputs (probably the non-inverted output, pin 2 of the XLR) and there is a second circuit that creates a single-ended inverted signal for pin 3 of the XLR. What's happening here is both signals reference ground.

The MP-3 doesn't do it that way. Its output, although direct coupled, acts very much like a simple line transformer with only 2 wires at its output- one tied to pin 2 of the XLR and the other to pin 3 with no connection to ground. So the advantage might be that the MP-3 can control the interconnect better, reducing artifact that the cable might otherwise have (and of course one of the advantages of that is you can run really long interconnects, allowing your speaker cables to be as short as possible; at home my amps sit on top of my speakers...). Of course if you have any other sources like a turntable or tuner then the advantages of a preamp are magnified. I use my DAC to accept HDMI for the home theater.