can someone answer me this.....

I make my own speaker cables

Oh dear. Sorry to be the one to red pill you, but there are no good DIY cables. Period. Whatever you spend on material will always buy you way better performance in a professionally designed product. Always. No exceptions. Not even close.

What you have right now might as well be lamp cord. No offense. It just is. Never heard any exceptions to this. A long time DIY'er one time compared his all-time best in my system. Even he had to admit it was handily trounced by some random thing I pulled out of my old cables drawer. About $75 from Synergistic, something like that.

You've got good components. Before doing anything else, get some good wire.

Regarding impedance compatibility, the DNA-500 has a specified input impedance of only 10K, and the manual for the ET3 recommends a minimum load impedance of 20K. Based on some interior photos I found on the web it appears that both the ET3 and the ET3SE utilize 4 uF coupling capacitors at their outputs, so the 20K recommendation presumably applies to the ET3SE as well as to the ET3.

However, given that the impedance of a 4 uF capacitor at 20 Hz is 1990 ohms, and given that the output impedance of the preamp at frequencies above the deep bass region is undoubtedly far lower than that (the specified nominal output impedance is 100 ohms), while the resulting mismatch will produce a slight rolloff and a bit of phase shift in the bottom octave, that is unlikely to be the major contributor to the problem. More likely the single tube that is used in the unit (assuming the optional phono stage is not included) is the major culprit, as Ralph (Atmasphere) suggested. And if the optional phono stage is included, given that you are using a line-level source the single tube that is used in the preamp’s line section is what should be focused on.

Regards,
-- Al

Avoid new manufactured tubes! Stick with NOS vintage tubes. Especially small-signal types like the 12A*7 and 6DJ8/6922 types! These front-end tubes are very critical to SQ in preamps and power amps!

Toss a Siemens 6922 in there-- one that you have had tested and know it is good.  For $100, you should be rocking.

MillerCarbon.....I have not found that to be true.   The connecting ends are the most important (I've tried Oyeida and Furutech.....both good, but I like top Furutech)