there's a reason the mantis is popular - it's a bloody good case and reasonably priced!
I have one that I use mostly for video synthesis and it's great for this - if a little under-powered on the -ve rail for video (video uses a lot of -ve power) but the point here is that the psu is incredibly good - very quiet up to video rates (MHz) and not just the KHz needed for audio - I've been using mine daily for about 3.5years and have moved it around on on trains and planes a fair bit in the also reasonably priced cases - the newer one is significantly better than the old one! - to be honest I rarely notice the case as it's usually patched!!
another option might be a doepfer lc9 - again solid power - but might be a bit under-powered for 9u if using a lot of digital modules
re:maths - yes you could patch everything from other modules - but it's really convenient to have it all in one module - I'm keeping my maths - and I have all the functionality duplicated a few times in my rack - but it is significantly bigger than just the mantis!
"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia
Utility modules are the dull polish that makes the shiny modules actually shine!!!
sound sources < sound modifiers < modulation sources < utilities