From experience I agree with Mike Creek in that an amp that has class A drivers with class AB outputs is certainly a legitimate design topology. I've used more than one solid state amp product that works this way & they sound just fine to me, and I am quite particular about sound quality. Of course a pure class A design is going to have a more purified sound than the hybrid design, but the tradeoff is a lot of wasted electricity & a hotter listening room. This extra heat isn't really an issue during cooler seasons, but on warmer days you'll certainly notice higher AC bills &/or a hotter house. Simply let your ears decide which you most prefer.
The amp doesn't normally "switch" back & forth though, this is simply driven by the input signal level exciting whichever stages at any given moment in time. One exception is the old Carver commutating power supply, but I wouldn't recommend that deisgn for any critical-quality applications.
Switching power supplies might be found in either type of design, & yes they can be noisy (although more efficient). In a properly designed component, either linear or switching power supply could sound equally good. I believe that Meitner, Rowland, & more recently TACT have manufactured examples of properly designed & executed "switchers".
