Peachtree audio iDecco - not hearing any difference with the tube on or off.

So I bought a used iDecco and I am using it as a DAC preamp. It is connected to an Onkyo equalizer via its Line Out and the equalizer is connected to my Artcoustic PA300 power amp. I have no complains about its sound as is, I am extremely happy with my current setup, But I noticed that the tube does not really have any impact to the sound? Is it because I am using the iDecco as a DAC and not as an integrated amp? I havent had the chance yet to connect the speakers directly to the idecco and test the tube. I also just recently changed the tube from 6nj1 to Amperex orange label ECC88/ 6DJ8. I just cant hear any difference at all whether the tube is on or off.