The 175 challengers are only taking place , as far as I can tell in the second week of the new 10 day masters events , where the masters are back to back ie week 2 of IW (just before Miami) and week 2 of Madrid (just before Rome). The events attract good entries as the players are all in the same locale and enter so that they can play an event of stature if theyve lost early in the masters event. So the entries tend to be stacked with top 100 or top 50 players, although many drop out as the events get closer, if they are still in IW or Madrid.
Aix as an event is also one of the oldest and best established challengers anyway, draws good crowds and a strong entry , but I think this is its first year as a 175. I think there are only this and the one in the USA alongside IW, I could be wrong on that though !