The SB15CAC is one of the best budget drivers out there but also probably the most expensive on the typical list that would still be called a “budget” driver. After that you are getting into Scanspeak and Seas, etc. SB15CAC will go lower than the MAC-05, for box modelling recommendations look at the SB15NBAC-8 at Madisound (they don’t have the box recommendations for the CAC versions or the NBAC-4).
The MAC-05 is unfair as a comparison because it is sold well below market price to raise funds for MAC. I would say it would be in the $45-$60 range like the SIG150 or RS150 if it was sold at PE or Madisound. I would guess that the SB15CAC is technically better, but not twice as good and surely not 4 times as good. It would be a tough choice between MAC-05 at $65 and SB15CAC at $105. If you are trying to get the most bang-for-the-buck, MAC-05 all the way. If you are trying to get the absolute best performance within the budget, and being at the top of the budget is fine, then SB15CAC makes sense.
As a disclaimer, I think my speaker I am most proud of is the one that uses the MAC-05. Not because it is my “best” speaker, but because it is such a great “value” speaker. If you want some SPL and distortion measurements on a real baffle, I can post them.
I have seen posts in the past that designing a waveguide for the Corundum tweeters has proven difficult, although there is one on diyAudio for the 32TX (edit: see wolf’s post below for that WG). I have the NE149’s and am really thinking that with a waveguided 25TX, that would be one heck of a speaker.
