Suggestion please: How to improve sound.

As a new comer to audio world, I gradually build my system:

Theta Data II transport.
Theta Pro Basic II DAC.
Audio Research LS-15 tube preamp (superb).
Aragon 8008 Mk2.
NHT 3.3.

All cables are Monster's at this time. For speaker cables, DIY cat5 is on the way. I am using gauge 12 generic cable now.

This system sound pretty good to me. However, I feel it should sound better. The only A/B test I did is to compare preamp to Myryad MP100 and Threshold F10, LS-15 sounds absolutely better, smooth and more musical. I also compare Aragon amp to Threshold S300 and seems NHT 3.3 likes power.

Overall, I feel this system could sound more transparent and more airy. Since I have no chance to do many A/B test to pickup the best equipement, I would like to hear some comments from real audiophiles here. Which is bottle neck in my system?

But I don't have too much money to burn. I once heard Pass Lab X350 which superb, but it is just too expensive. I was thinking about Aragon Palladium. I wonder if upgrade from 8008 to Palladium with more power will improve a lot?

Any comments are welcome and thanks in advance.

Roy