Wow - those are really small openings. You are going to have a hard time finding speakers that small that put out much sound. I would stay away from the BIC or the Pyle, especially the Pyle. Obviously that 400w rating is pure fiction. Some thoughts:
A 5 1/4" woofer should be minimum but none may fit - you might need to go to a 4" model
Look for a wood or metal cabinet - more rigid / usually sound better than plastic speakers
You may need to look at metal indoor / outdoor speakers - they tend to be smaller
If there is a port, it should face forward so the sound isn’t trapped behind the speaker
Bass will require sub - for a super budget sub look at Amazon's Polk subs - OK for $100
Originally Posted by Szumaj
Which of the below would be the loudest (without damaging anything) when linked to my receiver in a pair of 4? And what exactly is 2 way (MTM)?
The loudest speaker will have a higher efficiency / sensitivity - higher the number the louder the speaker (if you can believe the ratings). 2 way means there are two drivers which handle two different frequency ranges - in this case a tweeter for highs and a woofer for mids. MTM is a speaker with Mid - Tweeter - Mid arrangement of drivers (D'appolito speaker) – those aren’t going to fit those openings either.
This is the most sensitive speaker you linked that also has a front port - but according to your measurements it won't fit:
MTX Monitor 5i 5-1/4" 2-Way Bookshelf Speaker Pair
Sensitivity: 88 dB 2.83V/1m
Dimensions: 10 1/2" H x 6 1/2" W x 8 1/2" D.