IMHO, it's not necessarily a good thing. As an example, Da Hong Seetoo used to modify Jadis gear, and while he made it objectively better from a measurements standpoint, some of us felt that the mods robbed us of the Jadis house sound that we wanted. I believe one of our members here on A-gon actually had the mods undone in a piece he had gotten. Others, of course, liked the sound he created, so in large part it was a personal preference issue.
As another example, my prior speakers seemed like a good candidate for upgraded crossovers, so we built a standalone crossover box and put far better quality parts into that crossover. However, the chokes we used were so much better that they had a different resistance value (even though they were rated the same) and it made the speakers unbearably bright. Fortunately, the designer of the speakers was a member of our audio club, so he came over and redesigned the crossover to accommodate the new parts. The end result was a much better-sounding speaker (to my ears), but it took a lot of work.
Ultimately, if someone has modified their gear, I'm sure they did it to sound better to them (and it indeed might sound better on an objective and subjective scale), but in that case I'd still want to hear it myself before buying it, because it may not necessarily sound the same as the brand name on the equipment.
As another example, my prior speakers seemed like a good candidate for upgraded crossovers, so we built a standalone crossover box and put far better quality parts into that crossover. However, the chokes we used were so much better that they had a different resistance value (even though they were rated the same) and it made the speakers unbearably bright. Fortunately, the designer of the speakers was a member of our audio club, so he came over and redesigned the crossover to accommodate the new parts. The end result was a much better-sounding speaker (to my ears), but it took a lot of work.
Ultimately, if someone has modified their gear, I'm sure they did it to sound better to them (and it indeed might sound better on an objective and subjective scale), but in that case I'd still want to hear it myself before buying it, because it may not necessarily sound the same as the brand name on the equipment.