Quoted from nosro:
The main constraint is to buy a speaker with a mounting depth no more than approximately 2 3/4", at which point the speaker would rub against the playfield top edge in its raised position.
Yes, watch for this ^^^^. I had installed an upgraded sub kit from PinballPro in my High Speed and it was so tall that I could not bring the playfield to its raised position. However, this was YEARS ago so they may be using a lower profile setup now.