I am trying to pick out the speakers for my inwall surrounds for my 5.1 set up. I have narrowed it down to speakercraft the Three line but am unsure of how much base I need them to be able to handle.

I am look at the MT8 Three and the MT6 Three, the main difference being the woofer and therefore the low end. The MT8 Threes go down to 39Hz where as the MT6 three goes down to 40Hz... On paper not a big difference but in real life the MT8 did have a good amount more low end...

Your ear can not tell where base is coming from and if I have my sub and an L&R floor standing which reach 20Hz do I need the added base of the MT8 coming from the rear or should I save the money and get the MT6... FYI I thought the mids and highs where identical for each and the MT8 is nearly 3 times the price.

the installation of either speaker will make the most difference on the sound. Since they are and open back design and not a closed "chamber" design, the sound(specifically bass response) will depend on how much room they have to fill when installing them. Thats why i like in-wall/in-ceiling speakers such as monitor audio, new totems, triad, etc. that use an enclosure behind the speaker so they sound as much as they did at the factory as they can.

To answer your question between those two at moderate volumes, and as surrounds(which you will likely set to "small in receiver/pre-pro) and will be cut off at 80 hz anyway..... i just doubt you will be able to hear much of a difference if any