Firstly - tidy job there....all very neat and pro-style !
Adding bass by increasing the coupling caps can, in my experience, take you down a very mushy route - often as not I've found the tone changes a lot when you allow more bass. YMMV of course - some add component sockets to you can try different coupling caps to see which sounds best.
Polarity inversion can be performed with a single transistor stage wired as an inverting buffer (I'd try an Alembic Stratoblaster as a good simple design - I'm pretty certain it inverts...someone will be me straight if not)
HtH
Edit: One thing I've been tempted to try instead of a blend, is to ape the Boss LS-2 and rather than blending, having two volumes for the clean and effected. I've had variable results with blend circuits before.