budget diy tube preamp beating many expensive preamps... problem with my system?

It's not the cables, it's the simple circuit design and the use of the 6v6 tube. These types of tubes in a preamp give a big bold sound with better air and instrument separation in my opinion.

@invalid what would be an upgrade from this then

I will eventually get better cables. Tbh the Amazon rca cables I have seem better built than my custom tedd allen cables.

It is a question of values. What you call good may not be what I call good.
 

What I call good has changed over the course of the fifty years I have pursued high end sound. Slowly with experience I was able to listen deeper into more aspects of sound. Then about twenty five years ago I began to activity pursue listening to as much live an acoustical music and individual instruments, and my values changed considerably. Before this, upgrades in my system would make one kind of music sound better and all the rest would sound worse.
 

I realized that accentuated details were getting in the way of accurate representation of the music. Finally, I got what rhythm and pace really was… the attribute that gives the music the emotional connection and makes you want to move. All this ultimately brought me to all Audio Research equipment.

YOu do have some red flags in your system, especially the power strip.  I doubt you are hearing anywhere near the capability of your amp.

that said, a simple tube system will often outperform a complicated integrated circuit with swiss army knife features.

Jerry