Is something wrong w/ my pre???

I recently purchased a Conrad Johnson PV10AL and hooked it up to a Yamaha MX-1 amp, Sony DVP7000 (temporarily serving as a CD player) and Paradigm minis. Sounded OK - that's it. Maybe this result was caused by the amp, CD player, &/or even speakers. Well, soon after this discovery I wanted to see if there were noticeable sonic differences between the CJ and a Yamaha A/V receiver I had. I did an A/B test between the CJ and receiver (pre-outs) -- everything else remained constant. Unfortunately, what I discovered after the tests was that no significant improvements could be heard when listening to the CJ preamp. In fact, differences were nearly inaudible. How could this be? Everything in the chain was the same when performing the test between the CJ vs. the Yamaha AV Receiver. How could there be hardly any difference? Could the tubes in the CJ be going bad??? Even with only 400 hrs playing time? Am I missing something here? I'm only seeking answers as to WHY there was hardly any difference.