Strangely enough, many speakers that will do a fairly good job with most music simply do not do a good job with the human voice. This can even happen with some fairly expensive speakers, which is why one should always use CDs with male and female singers for auditioning speakers. Solo violin recordings are also sibilant in most cases and make a good test.
The problem is usually mainly in the speaker, but cheap amplifier circuits, which most HT receivers unfortunately have, definitely can make it worse. Driving the speakers harder is definitely going to make it worse, both in the amplifier and in the speaker.
One of the reasons I recently bought a new set of speakers was this very issue. My old speakers were not the worst by any means (highly rated and not cheap at all), but my new ones are the best I have ever heard (under $5000, that is), and I just had to have them because they are so good in exactly that respect. They are Gallo CL-3 speakers, and they are incredibly good in all respects, but especially from that standpoint.
Crossover networks are often part of the problem, causing phase problems in the voice region. One unique thing about the Gallo speakers is they use super-fast carbon-fibre drivers that go from 40 Hz up to 3 KHZ, well above the voice region, so there is no crossover network at all in the speaker system. The tweeter is also unique and has a LF limit of 3 Khz, so, again, no crossover is used.
There were certain movies and TV shows that were extremely difficult to understand before (Foyle's War, for one...), and now the sound is very clear and intelligible; an amazing difference.
I do have a very high-quality preamp and amplifier, so that is a non-issue in my case.
Edited by commsysman - 6/24/12 at 5:59pm