ProAc or Harbeths?

Boils down to listening room size, preference , type of music you play with.
Best if you can audition both as both will have their respective strengths & weaknesses.
Different people here will have different preferences & different opinions, in the end, what matters the most is what you prefer.
Shortside of things: I heard both in different systems & both sounds very good!! take your pick, BMW or Mercedes ? or Toyota or Honda ?

Bass of these two speakers are different.
The D38 has a tighter bass but the M40.1 has a fuller, loose type of bass.
The M40.1 has a fuller midrange compared to the D38 for that matter.
The D38 has a more airy, more detailed sound.
The m40.1 needs to be matched w/ the room as this can easily be boomy, D38 not as critical.
Its best if you can hear both side by side as they have a different presentation. Which one you prefer is something you can determine w/ proper audition.

Luna, stay with the Harbeth SHL5s. They are very good speakers & as you upgrade your electronics, the speakers will let the differences shine thru.