You seem to have a pretty solid starting point, a 41 in quant is nothing to scoff at. To push past and into the 45+ range you need to know the basics and be able to do questions where multiple concepts are tested.
Stick to the MGMAT material to build a solid "GMAT" specific foundation towards quant and verbal, don't bother with Kaplan Premier
The math concepts start to get intertwined and multiple concepts are tested on the higher level questions. Don't expect MGMAT material to be the end all, know all for Quant. They do a fantastic job of introducing each concept but don't throw multiple concepts at you in a lot of their questions (in the books atleast, practice exams a different story). The books will help you build a foundation but after that start to do a lot of problems and note down how individuals concepts are mixed in to test your knowledge: values given in percentages in overlapping sets, comb & permutation to solve probability questions, switching between different sets of units for rate or work problems, ratios in stats/average questions, etc.
How to spot and work with multiple math concepts will make life easier. It's kind of like how GMAC likes to throw in multiple errors for the harder sentence correction questions.