He says the improvements he made will be easy for me to hear. They also sound like what I was hoping would improve.
I am looking for more clarity and detail. A bit quieter would be nice, but the new LA4 preamp I am getting may solve that issue. More musical engagement
or the wow factor would be the best improvement.
Clarity and detail improvements would require reductions in distortion. The distortion of this amp is probably already below the threshold of audibility. Anything he does is likely to decrease performance. Does he have ability to measure before and after to provide any sort of proof that he has actually improved the performance of the amp? Let me answer: No. It will just be his subjective opinion.
More musical engagement and wow factor? How to do quantify these? You can't. It's subjective. He will monkey around with the amp and add noise to it. You can use software to add effects and tune it to your own specific likes easier, cheaper, and with much more control.