I would have a competent ac shop check the ac system, it may be low on freon, the radiator and condenser may be dirty and impeding the low speed air flow, the blend door in the box may not be sealing tightly, just some things that could be causing a low speed problem, at one time this system worked good as designed so first look at what may be worn/miss adjusted or leaking is the first thing to do rather than add aftermarket upgrades that will make finding the problem harder to fix. JMO